If i understand correctly, Discord is a chat application for private and group chats. There is Matrix chat that works reasonably well as a replacement.

It’s decentralized, open source software, and i’ve used and tested it for at least a year now. It works well enough. It does not use the ActivityPub protocol though.

Mind that Matrix chat tends to private/group chats, which means they’re not typically visible to everyone else. Which means it would make little sense to try to embed them into the rest of the fediverse, which is openly accessible.