Mumble would be my bigger recommendation for a truly open source Discord alternative, though I'm personally more invested in XMPP as an alternative.
replyMumble is fantastic for voice chat. Its text features are very basic, though, so people fleeing Discord would probably want something additional to handle that. Maybe Matrix.
replyA single location is a good selling point. Being able to jump into a voice chat, and still post things in a shared text chat is a good feature. Mumble should work a bit on that.
reply> Mumble should work a bit on that.
replyMumble is a labor of love, not a commercial product. I expect they would appreciate your help.
> self hosted servers register with a centralized server to check for a license and to optionally list that server in the centralized list of public servers
replyI doubt license authorization and an entry in a list are overwhelming their servers.