The Clojurians Slack -- self-signup: http://clojurians.net and chat: https://clojurians.slack.com -- is the largest online community of Clojure developers right now (January 2021). Many other online communities for Clojure exist: starting with r/Clojure on Reddit and that lists most of the other popular hangouts in its right hand navigation bar (we don't duplicate them here so that we don't have to also keep a list up-to-date here!).
The Clojurians Slack operates under this Code of Conduct.
We've historically used the free plan for Slack because of the large number of active users (it would cost many thousands of dollars a month to upgrade), which meant that only a few days of messages are visible/searchable. We currently have a sponsored "Pro" plan courtesy of Slack themselves, but that could change in future. With the Pro plan, we have all our history available and searchable.
Most channels are also archived to https://clojurians-log.clojureverse.org/ (and if you create a new channel on the Clojurians Slack, you can /invite @logbot to it in order to log messages to that site). Many channels are also archived to the Clojurians Zulip Chat which is on the free open source plan and therefore has unlimited message visibility/searchability -- Slack channels show up as "topics" under the "slack-archive" stream (and if you create a new channel on the Clojurians Slack, you can /invite @zulip-mirror-bot to it in order to log messages to the Zulip chat).
For more details see Andy Fingerhut's write up about archiving and searching those archives.
This repo also includes historical notes about Slack alternatives that were discussed as part of the old Hackpad documents.