Forums: great store of knowledge and friendly, helpful people. If you ask a question in discord, nobody will ever see the answer again.
The search functions in forums are notoriously terrible though (although someone will inevitably ask you to try using it), so finding anything useful relies on "outside" search engines.
And the linear thread format has been terrible since it was invented (which is probably why discord uses it). You basically need to ignore half the posts to follow the one interesting side line that might end up with a solution.
Because setting up an IRC server is way, way easier than setting up a matrix server. It's also a lot more reliable. The downside is that it's text only