It’s totally possible to make a true 1-1 competitor to Discord but no one has done it yet.

Nextcloud Talk is probably the closest thing with federation. But Nextcloud is notoriously complicated to host. And for some reason super unpopular outside of Europe.

Stoat (formerly Revolt) is very close, both in functionality and UI. But is not federated (and has no plans to) and for some reason not very popular. Also doesn’t come with the wealth of configuration and moderation tools that Discord does.

Matrix works but it’s super slow, bug-ridden, and complicated to use.

Zulip is similar but I’ve used it fairly extensively and still have no idea WTF is happening. Also not federated.

Of course there’s always IRC and whatever other ancient comms tools that still work fine but are very dated and lack functionality.