Zulip. Open source, self-hostable, looks decent and not only does it have every feature that discord has, their version of threads/topics to spin off individual conversations is extremely well done.

And, don't overlook the indexability of Zulip so that information doesn't instantly /dev/null just because Discord hates having bots or because one can only ever(!) join 100 Discord servers

e.g. https://servo.zulipchat.com/#narrow/channel/416780-feature-t...