Then I must be missing a lot of projects, because I don’t know of any which use discord for any of these things, besides questions and answers. And even then, only for informal stuff. Anything more serious goes on GitHub (or alternatives) or forums.

Public forums serve a different purpose to a live chat. Actually what I’m seeing a lot in the comments is people making assumptions about what the purpose of the discord server is, and from those assumptions then conclude that it would be easy to use something else. Some assume that they use it for documentation (???), some assume it’s for technical support. But usually what I see, is that these kinds of servers are just an attempt to build a community. And for building a community in the tech space, discord has by far the biggest user base for a chat solution (unless you really think there are more target audience using Facebook messenger, than discord?)

I’m sorry but where is this random assumption that the documentation is on discord coming from?? It is extremely common for projects/organisations to have some kind of community discord, but I have never seen one that used it as their main documentation host. The discord are almost always just community hubs to chat with other community members.

then don’t brag about being 100% FOSS supporter

You are literally doing exactly what the meme was about lol. A discord alternative may simply not be a viable alternative for their needs, if no one is using it.