Slack and A Forum are not comparable. Slack is largely for real-time communication which is rarely referenced historically, a forum is more of a knowledge store.
Everything is comparable! Some comparisons don't make sense because of how different they are, but Slack vs eg Discourse (not Discord) are in a similar enough realm of human interaction that it's worth taking the time to make the comparison. Especially because Slack is seen as the be-all end-all of where things live these days.