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. [1]: https://once.com/campfire
The owner's terrible reputation, no mystery here.
However, the owner (37signals, not DHH) has an excellent reputation for producing high quality web applications since the early 2000s.