diff --git a/changelog.d/17282.feature b/changelog.d/17282.feature
new file mode 100644
index 00000000000..334709a3a75
--- /dev/null
+++ b/changelog.d/17282.feature
@@ -0,0 +1 @@
+Include user membership in events served to clients, per MSC4115.
\ No newline at end of file
diff --git a/docker/complement/conf/workers-shared-extra.yaml.j2 b/docker/complement/conf/workers-shared-extra.yaml.j2
index 4c41ee77092..6588b3ce147 100644
--- a/docker/complement/conf/workers-shared-extra.yaml.j2
+++ b/docker/complement/conf/workers-shared-extra.yaml.j2
@@ -105,8 +105,6 @@ experimental_features:
   # Expose a room summary for public rooms
   msc3266_enabled: true
 
-  msc4115_membership_on_events: true
-
 server_notices:
   system_mxid_localpart: _server
   system_mxid_display_name: "Server Alert"
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index b306b807490..4ad547bc7e5 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -223,7 +223,6 @@ test_packages=(
     ./tests/msc3930
     ./tests/msc3902
     ./tests/msc3967
-    ./tests/msc4115
 )
 
 # Enable dirty runs, so tests will reuse the same container where possible.
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 542e4faaa1d..9265a271d23 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -238,7 +238,7 @@ class EventUnsignedContentFields:
     """Fields found inside the 'unsigned' data on events"""
 
     # Requesting user's membership, per MSC4115
-    MSC4115_MEMBERSHIP: Final = "io.element.msc4115.membership"
+    MEMBERSHIP: Final = "membership"
 
 
 class RoomTypes:
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 5fe5b951dd4..d9ad5fc32d5 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -436,10 +436,6 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
                 ("experimental", "msc4108_delegation_endpoint"),
             )
 
-        self.msc4115_membership_on_events = experimental.get(
-            "msc4115_membership_on_events", False
-        )
-
         self.msc3916_authenticated_media_enabled = experimental.get(
             "msc3916_authenticated_media_enabled", False
         )
diff --git a/synapse/handlers/admin.py b/synapse/handlers/admin.py
index 21d3bb37f38..ec35784c5f5 100644
--- a/synapse/handlers/admin.py
+++ b/synapse/handlers/admin.py
@@ -42,7 +42,6 @@ def __init__(self, hs: "HomeServer"):
         self._device_handler = hs.get_device_handler()
         self._storage_controllers = hs.get_storage_controllers()
         self._state_storage_controller = self._storage_controllers.state
-        self._hs_config = hs.config
         self._msc3866_enabled = hs.config.experimental.msc3866.enabled
 
     async def get_whois(self, user: UserID) -> JsonMapping:
@@ -215,7 +214,6 @@ async def export_user_data(self, user_id: str, writer: "ExfiltrationWriter") ->
                     self._storage_controllers,
                     user_id,
                     events,
-                    msc4115_membership_on_events=self._hs_config.experimental.msc4115_membership_on_events,
                 )
 
                 writer.write_events(room_id, events)
diff --git a/synapse/handlers/events.py b/synapse/handlers/events.py
index 09d553cff1e..3f46032a43a 100644
--- a/synapse/handlers/events.py
+++ b/synapse/handlers/events.py
@@ -148,7 +148,6 @@ class EventHandler:
     def __init__(self, hs: "HomeServer"):
         self.store = hs.get_datastores().main
         self._storage_controllers = hs.get_storage_controllers()
-        self._config = hs.config
 
     async def get_event(
         self,
@@ -194,7 +193,6 @@ async def get_event(
             user.to_string(),
             [event],
             is_peeking=is_peeking,
-            msc4115_membership_on_events=self._config.experimental.msc4115_membership_on_events,
         )
 
         if not filtered:
diff --git a/synapse/handlers/initial_sync.py b/synapse/handlers/initial_sync.py
index 84d6fecf313..bd3c87f5f4e 100644
--- a/synapse/handlers/initial_sync.py
+++ b/synapse/handlers/initial_sync.py
@@ -224,7 +224,6 @@ async def handle_room(event: RoomsForUser) -> None:
                     self._storage_controllers,
                     user_id,
                     messages,
-                    msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
                 )
 
                 start_token = now_token.copy_and_replace(StreamKeyType.ROOM, token)
@@ -383,7 +382,6 @@ async def _room_initial_sync_parted(
             requester.user.to_string(),
             messages,
             is_peeking=is_peeking,
-            msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
         )
 
         start_token = StreamToken.START.copy_and_replace(StreamKeyType.ROOM, token)
@@ -498,7 +496,6 @@ async def get_receipts() -> List[JsonMapping]:
             requester.user.to_string(),
             messages,
             is_peeking=is_peeking,
-            msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
         )
 
         start_token = now_token.copy_and_replace(StreamKeyType.ROOM, token)
diff --git a/synapse/handlers/pagination.py b/synapse/handlers/pagination.py
index dab3f90e74a..872c85fbadd 100644
--- a/synapse/handlers/pagination.py
+++ b/synapse/handlers/pagination.py
@@ -623,7 +623,6 @@ async def get_messages(
                 user_id,
                 events,
                 is_peeking=(member_event_id is None),
-                msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
             )
 
         # if after the filter applied there are no more events
diff --git a/synapse/handlers/relations.py b/synapse/handlers/relations.py
index de092f8623b..efe31e81f98 100644
--- a/synapse/handlers/relations.py
+++ b/synapse/handlers/relations.py
@@ -95,7 +95,6 @@ def __init__(self, hs: "HomeServer"):
         self._event_handler = hs.get_event_handler()
         self._event_serializer = hs.get_event_client_serializer()
         self._event_creation_handler = hs.get_event_creation_handler()
-        self._config = hs.config
 
     async def get_relations(
         self,
@@ -164,7 +163,6 @@ async def get_relations(
             user_id,
             events,
             is_peeking=(member_event_id is None),
-            msc4115_membership_on_events=self._config.experimental.msc4115_membership_on_events,
         )
 
         # The relations returned for the requested event do include their
@@ -610,7 +608,6 @@ async def get_threads(
             user_id,
             events,
             is_peeking=(member_event_id is None),
-            msc4115_membership_on_events=self._config.experimental.msc4115_membership_on_events,
         )
 
         aggregations = await self.get_bundled_aggregations(
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 203209427b2..2302d283a74 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -1476,7 +1476,6 @@ async def filter_evts(events: List[EventBase]) -> List[EventBase]:
                 user.to_string(),
                 events,
                 is_peeking=is_peeking,
-                msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
             )
 
         event = await self.store.get_event(
diff --git a/synapse/handlers/search.py b/synapse/handlers/search.py
index fdbe98de3b0..a7d52fa6483 100644
--- a/synapse/handlers/search.py
+++ b/synapse/handlers/search.py
@@ -483,7 +483,6 @@ async def _search_by_rank(
             self._storage_controllers,
             user.to_string(),
             filtered_events,
-            msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
         )
 
         events.sort(key=lambda e: -rank_map[e.event_id])
@@ -585,7 +584,6 @@ async def _search_by_recent(
                 self._storage_controllers,
                 user.to_string(),
                 filtered_events,
-                msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
             )
 
             room_events.extend(events)
@@ -673,14 +671,12 @@ async def _calculate_event_contexts(
                 self._storage_controllers,
                 user.to_string(),
                 res.events_before,
-                msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
             )
 
             events_after = await filter_events_for_client(
                 self._storage_controllers,
                 user.to_string(),
                 res.events_after,
-                msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
             )
 
             context: JsonDict = {
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 39964726c5b..f1c69d98937 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -844,7 +844,6 @@ async def _load_filtered_recents(
                     sync_config.user.to_string(),
                     recents,
                     always_include_ids=current_state_ids,
-                    msc4115_membership_on_events=self.hs_config.experimental.msc4115_membership_on_events,
                 )
                 log_kv({"recents_after_visibility_filtering": len(recents)})
             else:
@@ -930,7 +929,6 @@ async def _load_filtered_recents(
                     sync_config.user.to_string(),
                     loaded_recents,
                     always_include_ids=current_state_ids,
-                    msc4115_membership_on_events=self.hs_config.experimental.msc4115_membership_on_events,
                 )
 
                 loaded_recents = []
diff --git a/synapse/notifier.py b/synapse/notifier.py
index ced9e9ad667..c87eb748c0d 100644
--- a/synapse/notifier.py
+++ b/synapse/notifier.py
@@ -721,7 +721,6 @@ async def check_for_updates(
                         user.to_string(),
                         new_events,
                         is_peeking=is_peeking,
-                        msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
                     )
                 elif keyname == StreamKeyType.PRESENCE:
                     now = self.clock.time_msec()
diff --git a/synapse/push/mailer.py b/synapse/push/mailer.py
index 49ce9d6ddac..77cc69a71f5 100644
--- a/synapse/push/mailer.py
+++ b/synapse/push/mailer.py
@@ -532,7 +532,6 @@ async def _get_notif_vars(
             self._storage_controllers,
             user_id,
             results.events_before,
-            msc4115_membership_on_events=self.hs.config.experimental.msc4115_membership_on_events,
         )
         the_events.append(notif_event)
 
diff --git a/synapse/visibility.py b/synapse/visibility.py
index c891bd845b3..128413c8aab 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -82,7 +82,6 @@ async def filter_events_for_client(
     is_peeking: bool = False,
     always_include_ids: FrozenSet[str] = frozenset(),
     filter_send_to_client: bool = True,
-    msc4115_membership_on_events: bool = False,
 ) -> List[EventBase]:
     """
     Check which events a user is allowed to see. If the user can see the event but its
@@ -101,12 +100,10 @@ async def filter_events_for_client(
         filter_send_to_client: Whether we're checking an event that's going to be
             sent to a client. This might not always be the case since this function can
             also be called to check whether a user can see the state at a given point.
-        msc4115_membership_on_events: Whether to include the requesting user's
-            membership in the "unsigned" data, per MSC4115.
 
     Returns:
-        The filtered events. If `msc4115_membership_on_events` is true, the `unsigned`
-        data is annotated with the membership state of `user_id` at each event.
+        The filtered events. The `unsigned` data is annotated with the membership state
+        of `user_id` at each event.
     """
     # Filter out events that have been soft failed so that we don't relay them
     # to clients.
@@ -159,9 +156,6 @@ def allowed(event: EventBase) -> Optional[EventBase]:
         if filtered is None:
             return None
 
-        if not msc4115_membership_on_events:
-            return filtered
-
         # Annotate the event with the user's membership after the event.
         #
         # Normally we just look in `state_after_event`, but if the event is an outlier
@@ -186,7 +180,7 @@ def allowed(event: EventBase) -> Optional[EventBase]:
         # Copy the event before updating the unsigned data: this shouldn't be persisted
         # to the cache!
         cloned = clone_event(filtered)
-        cloned.unsigned[EventUnsignedContentFields.MSC4115_MEMBERSHIP] = user_membership
+        cloned.unsigned[EventUnsignedContentFields.MEMBERSHIP] = user_membership
 
         return cloned
 
diff --git a/tests/rest/client/test_retention.py b/tests/rest/client/test_retention.py
index ceae40498e1..1e5a1b0a4d9 100644
--- a/tests/rest/client/test_retention.py
+++ b/tests/rest/client/test_retention.py
@@ -167,7 +167,6 @@ def test_visibility(self) -> None:
                 storage_controllers,
                 self.user_id,
                 events,
-                msc4115_membership_on_events=True,
             )
         )
 
diff --git a/tests/test_visibility.py b/tests/test_visibility.py
index 3e2100eab47..89cbe4e54bc 100644
--- a/tests/test_visibility.py
+++ b/tests/test_visibility.py
@@ -336,7 +336,6 @@ def test_joined_history_visibility(self) -> None:
                 self.hs.get_storage_controllers(),
                 "@joiner:test",
                 events_to_filter,
-                msc4115_membership_on_events=True,
             )
         )
         resident_filtered_events = self.get_success(
@@ -344,7 +343,6 @@ def test_joined_history_visibility(self) -> None:
                 self.hs.get_storage_controllers(),
                 "@resident:test",
                 events_to_filter,
-                msc4115_membership_on_events=True,
             )
         )
 
@@ -357,7 +355,7 @@ def test_joined_history_visibility(self) -> None:
         self.assertEqual(
             ["join", "join", "leave"],
             [
-                e.unsigned[EventUnsignedContentFields.MSC4115_MEMBERSHIP]
+                e.unsigned[EventUnsignedContentFields.MEMBERSHIP]
                 for e in joiner_filtered_events
             ],
         )
@@ -379,7 +377,7 @@ def test_joined_history_visibility(self) -> None:
         self.assertEqual(
             ["join", "join", "join", "join", "join"],
             [
-                e.unsigned[EventUnsignedContentFields.MSC4115_MEMBERSHIP]
+                e.unsigned[EventUnsignedContentFields.MEMBERSHIP]
                 for e in resident_filtered_events
             ],
         )
@@ -441,7 +439,6 @@ def test_out_of_band_invite_rejection(self) -> None:
                 self.hs.get_storage_controllers(),
                 "@user:test",
                 [invite_event, reject_event],
-                msc4115_membership_on_events=True,
             )
         )
         self.assertEqual(
@@ -451,7 +448,7 @@ def test_out_of_band_invite_rejection(self) -> None:
         self.assertEqual(
             ["invite", "leave"],
             [
-                e.unsigned[EventUnsignedContentFields.MSC4115_MEMBERSHIP]
+                e.unsigned[EventUnsignedContentFields.MEMBERSHIP]
                 for e in filtered_events
             ],
         )
@@ -463,7 +460,6 @@ def test_out_of_band_invite_rejection(self) -> None:
                     self.hs.get_storage_controllers(),
                     "@other:test",
                     [invite_event, reject_event],
-                    msc4115_membership_on_events=True,
                 )
             ),
             [],