@wolf480pl IRC is my favorite because you have clients like ii.
If you need encryption XMPP is ok. It's really heavy though so there are like two clients for Linux with support for modern stuff.
Matrix just freaks me out.
I still don't have any good replacements for Discord's VC. It's even better than what we use at work, they really nailed that. I've heard mumble is good though.
@wolf480pl TBH async realtime chat kind of sucks. I wish people used email more at work so I could go back and find things they said that I thought wasn't important.
@swiley yeah mumble is good for voice, but its text chat is pretty minimal and sync-only. It doesn't really have the concept of long-term channel membership, the way Discord has, so it doesn't blend the sync and async chat.
But if a voice chat is all you need, Mumble is great.