Slack (and other RTM platforms) is actually very good for live conversations and that's what forums are missing. People used to use IRC for that before but the times have changed and Slack is currently the king. Forums, on the other hand, are much better for structured discussions and long term accumulation of content.
At OneBar (.io) we're trying these two concepts together. The idea is that you can discuss stuff in Slack in the real-time while it's "hot", but later on the most important parts get accumulated elsewhere and are available for the forum-style discussion. We're not currently designing it for online communities, but we've been brainstorming on this case a lot internally. We could pilot something if there's a community manager here willing to collaborate.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.