I agree, I think forums and things like Slack/Discord/IRC is used for different kinds of communication. Forums are rather asynchronous, while the others are rather synchronous.

When a forum is used to help someone fix a problem, there is a communication thread that (as you say) can be searched and followed by other people. In things like Slack/Discord/IRC, this kinda of thing "disappears". Even if you have seachable logs, I think these are a lot harder to search than a forum that is more "structured".

I do believe there is value in having both forums and Slack/Discord/IRC.