Searching for a single Discord alternative may be asking the wrong question however. Discord itself is an extensive bundle of functions smashed together: real-time chat, persistent forums and documentation, voice chats, events and even games.
I think this is the important part that's missing from a lot of these discussions, including from users themselves looking for a new place to go. Some people use discord as an IRC chatroom replacement. Some use it was a small group text, essentially, between friends or co-workers. Some people use it as a Patreon perk to get access to a community around an artist and interact with that artist and their other fans.
And I'm in some "servers" of all of those. So anywhere that's using it as IRC can be replaced with XMPP or Matrix no problem. Or IRC, but with gifs. Cool. But my other group that hangs out in there async every day and the occasionally jumps onto an ad-hoc voice chat when people are available to game, or sometimes shares my screen so someone else can watch what I'm doing? None of those things do that. But mumble kinda does, but not in a persistent or integrated way. Mumble is a great way to talk, but an awful place to hang out. Jitsi does screen share, but is not casual and also isn't a good hangout.
And we could limp by with an XMPP room for chat and then a link to a Jitsi or Mumble or something when it's time to do something. But there's something tight about having the "just call" button right there, tied to the chat you're already in, and in being able to see "huh Alice and Bob are playing GAME right now. I should pop in!"
But if you've never been in a discord server like that, you make a recommendation of IRC or something, and a gaming friend group user checks it out and is like "this is even close to doing any of the things I need it to..."