[rvdginste]

I think forum/mailinglist and Slack/Discord/IRC are different kinds of communication: the first is rather asynchronous/offline while the second is rather synchronous/online. I believe they can exist next to each other within a project or community.

For keeping track of specific questions and how those can be answered, a forum is a lot better. But random chatter can also be useful and that is a better fit for things like IRC.