Comment by ksec

It doesn't go into issues which I think is the most important.

Slack / Discord being free?

Who is setting up the forum? Who is paying for it? Who does the maintenance? Who does the moderation with Spams?

Part of the reason why Discourse took over most of the forum for Open Source is that they are free for Open Source Project. And there are little to no maintenance involves.

Not to mention to this day, forum software still sucks.