This thread mentions 3 things as alternatives to Discord:
Discourse (with guide to using it), phpBB, and Flarum.
Neatnik (@neatnik@social.lol)
Discourse is another excellent self-hosted alternative to Discord. I’ve just published a guide on how to get it up and running on your own server for less than the cost of a Discord “server boost” (bleh). https://www.neatnik.net/setting-up-a-discourse-server
social.lol (social.lol)
