I'm a little surprised nobody has mentioned Campfire [1].
It's open source, trivial to self-host, and can support an arbitrary number of rooms and users.
Sure, it doesn't have all of Discord's bells and whistles (for better or worse), but then neither do some of the alternatives mentioned in the article.
I've been on the lookout for Discord alternatives for my friend group for a while, and this is the first one that really looks promising to me. All the others are too expensive (Mattermost, Rocket) or introduce too much friction (IRC, Matrix). This looks like it could work!
I've been running a campfire chat with a few hundred users for a couple of years now. It's one of my favorite pieces of software simply because I don't spend any time thinking about it.
It's open source, trivial to self-host, and can support an arbitrary number of rooms and users.
Sure, it doesn't have all of Discord's bells and whistles (for better or worse), but then neither do some of the alternatives mentioned in the article.
[1]: https://once.com/campfire