I get GitHub - it's proprietary, but also just git, so moving somewhere else is just a pull and push away. The same can't be said for issues/discussions/wiki, but at least it's still the web, so it is googleable, archivable, curlable, you name it. It has happened more than a dozen times that I googled a weird error from eg libssl, ended up in a random issue of a random tool I never heard of, but someone still explained the underlying issue well enough while debugging that I was able to solve my problem. This requires a browser and nothing else.

None of this is true for discord. It isn't indexed by anything, even "servers" are walled gardens to each other mostly. Anything shared on discord will be lost to time in a decade or two. Nothing discovered or solved within $project's discord will possibly benefit anyone outside that project.

And sure, we already had that problem with IRC, but I think the text-only nature of it helped that anything a bit more involved moved to forums or mailing lists which again both were web accessible and searchable. And even then some projects did have logbots which just dumped text files into a web server.