Stoat (formerly known as Revolt) is a selfhostable, FOSS replacement for discord [Group chats and voice channels you can join any time].
Cool new name, however not as easy to use in other languages.
Voice chat is stil not officialy implemented.
Self-hosting there. Apparently nothing to do for you if you had already hosted before the name change.
The Android app has unfortunately disappeared (not been updated) on F-droid.
Edit: added short description for clarification
Syntax highlighting for code blocks is the reason I prefer discord over slack for collaborating and just chatting with friends who know how to code. I imagine some irc clients exist that so the same, but at least with discord I know my recipient is guaranteed to see what I see.