I've been part of an open source project for well over 2 years now. We use Slack for community management (3k users). It's pretty clear to me that Slack was never really meant to replace forums:

- Lots of the previously answered questions are lost

- Search is not great and is limited to Slack. We have many non-slack users searching through Google.

- It's noisy with random chatter

- It's harder for us to keep track of questions to answer.

I think Slack is great for really small communities or for new products that need quick feedback but not for large community management.