Comment by kelnos

I agree, but I don't think those things serve the same use cases, and I know some projects that have both a Slack workspace and a forum.

A forum is great for getting technical support, and is ok-ish for technical discussion and planning, but I don't think it's a good replacement for real-time chat, which is great for developing more nuanced, personal relationships (for distributed developers and users that aren't going to do things like regular video chats). Certainly real-time chat isn't going to give you as rich an experience as a video or in-person chat, but it's much better than the asynchronous back-and-forth of a forum (or email).

So I would say "Use IRC/Matrix rather than Slack/Discord to support developer community" would be a better position to take (and set up a bot to automatically archive conversations to somewhere searchable on the web), as well as "Supplement your real-time chat with a forum or email list" (which can also be searchable). On top of that, know where to capture information for the longer term: for example, if discussion on IRC or the forum leads to an understanding that the project's documentation is confusing in some areas, write up a ticket on GH Issues or Jira or whatever you're using. That way you (or someone else) will remember it needs to be done.