Having said that in many contexts a forum with threads that have a clear topic are just so much more of an appropriate solution than chat apps. I don't understand why people prefer the latter (although it does explain why I prefer Zulip since it manages to be a good hybrid in how it organizes discussions).
That means that they won't be indexed by search engines, so I cannot serendipitously stumble across them during a random web search in the first place, and even if I somehow learned about them, having to sign up merely to read some stuff is annoying.
I don't really like Discord, so I don't use it, and have no feedback on it.
I used to run a SimpleMachines forum, and we actually wanted to prevent the contents from being indexed.
Arc was released a few years after Reddit announced their Python rewrite anyways
They are tremendously useful still, and they also contain a lot of esoteric domain-specific knowledge you can't find elsewhere.