I know we’ve had this discussion before, but just to get it in the record, it’s far more than that. Discord functions very differently from Gitter in ways which foster community discoverability and interaction. This isn’t just an opinion, it’s been born out in hundreds of thousands of communities across innumerable topics.
Even if you don’t buy the “Discord works better for the users” argument though, the mere fact that it has moderation tools which work is reason enough for any community to switch. Even banning doesn’t work correctly on Gitter (you can just delete and recreate your account to bypass any ban or moderator action, which has been demonstrated in the Scala channels several times in the past). It doesn’t have gradient roles. It doesn’t even have a slow mode or other “table stakes” type tools for putting out fires. Discord has all of this, and given the communities it was originally built for (and continues to serve), it’s safe to say those tools are dramatically more battle-hardened than literally any other system in existence.
Discord vs Matrix is the only question that even bears examination here, since Gitter is simply not a tenable tool for any modern community on moderation grounds alone. Matrix is quite good, but it’s also very immature, lacks (and probably will always lack) a lot of the tooling that Discord has built up around it, suffers from the same discoverability problem as Gitter, and still has the walled-garden issue of Discord. The fact that Matrix is federated in theory doesn’t make it any more federated in practice than XMPP was (or is), and we all know how that saga ended.
Yes, it sucks that Discord is not indexed by Google. It is quite nice though that it’s indexed at all (Discord’s search is incredibly good), which is something that Matrix and Slack still can’t seem to get right. And at the end of the day, the concern of locking off community content behind a walled garden is only a meaningful concern if there exists community content to lock off, which is to say, an active community producing said content. Discord demonstrably fosters this better than other systems, and that’s reason enough to use it.