I used to self-host TS for my gaming buddies, it was amazing.
Mumble is exactly that but fully FOSS (unlike TS)?
Fair reason to glaze it IMO.
I don’t know, their landing page doesn’t showcase anything besides it being done right!
Maybe it’s great but they don’t sell it super nicely is all I’m saying.
All the solutions mentioned below are listed here pretty much.
They will all have their pros/cons and nobody will easily agree on a specific one because of everybody’s taste and because they do not remove enough friction as a whole.
Discord was successful for a few reasons but the main one is the fact that with one single account, you could quickly join any server in no time[1].
Very sorry about that one, I do understand the pain myself too. 
I guess we need to stop looking for a replacement of Discord and rather look at some different with its pros/cons. Otherwise it will be a mental burden. 
I haven’t tried all the mentioned solutions[2] below but having something public like Discourse is quite a must IMO: transparency, SEO etc. Again it depends on the needs yeah but it also having a chat feature as mentioned below is quite cool yet less hacker with a dark/gamer-y UI to it. 
Tbh, Discord also took years to be popular and de-facto the default. But yeah, it’s annoying when companies do those kind of moves that force people to move elsewhere.
At some point, maybe people will realize that locking themselves into a private platform is maybe the issue?
But I am overall not too optimistic about that mindset shift. 