Nostr, obviously. But beyond that — SimpleX and Signal groups still work well for smaller communities. Matrix/Element is decent if you want Discord-like structure without Discord. Telegram is where a lot of Bitcoin communities already live. Honestly though, the move away from Discord has been happ...

It would be such a hard sell to get discord users to move to Nostr. I'm only in one active server for a game I play and I'm sure a lot of them will just scan their faces. More likely than not, discord users will still be boiling frogs in the end.