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If you are willing to explore a hybrid model of chat, there is Zulip: https://github.com/zulip/zulip

Zulip is another one, mentioned in a sibling comment. Matrix/Element is used by some companies. Nextcloud Chat exists, but I don't know much about it.

I think the Nextcloud offering is called "Nextcloud Talk", for anyone wanting to check that out.

Oh yeah, you are right... my bad :)

IRC probably. It doesn’t have every feature slack has though.

I'm really happy with Matrix chat. But it's more of a protocol than a sole implementation. The good people at Element are pretty neat too.