The distinction isn't always clear. If you're studying the history of the Linux kernel, you'll want to make use of its mailing list archives.

If that were true than IRC would be absolutely perfect.

SerenityOS used IRC but when Andreas changed to discord he saw a massive influx of developers. I assume that it was a no brainer to use discord when forking Ladybird...

So would be Discord, and so it is.

Come on, it's inevitable that chat history becomes a knowledge base to some extent even if the participants are aware of this pitfall.

Only if you let it.