Matrix, what seems to be the most ovious replacement to Discord, is an incredible piece of software from a technical point of view. It have Conflict-free Replicated Datatype, which give an hard guarantee that no message will be skipped over and old message will also be fetched. Something that ActivityPub doesn’t permit, and is quite a problem with Mastodon at times (much less on lemmy, given you follow communities, and so everything on these communities will be synced, thought not backfilling)