@HAWK what is said is very true about slack and forums. Nowadays the usage of slack as a tool to maintain the community (mostly by technology & design communities) had increased drastically. But still slack can't take over the advantages of a forum.
In Slack, it promotes quick and fast engagement but fails to maintain the chronology of discussion. Slack is more a single threaded discussion platform. But the best part of slack is that it's a good tool to talk to everyone in the group and interact privately, Which helps in building networks, finding mentors, etc. Also, slack is handy for developers/designers because he/she will be online most of the time and easy access to the updates.
Forum in the other hand its multi-threaded and promotes deeper engagements than slack. Multi-threaded discussions help the member to focus on a particular topic that he's interested. The forum also provides activity log and stats of each member and which help us to identify the pro-active members which are good for the growth of the community. wherein slack this part is still void.
It's very hard to conclude which one is good for a community. Where Slack help to improve networking and relationship and forum helps to learn and solution seekers. But to select which one should use completely depends on the vision and the mission of the community.
This is my quick and vague analysis. What do you think?