I am not saying that Slack isn’t great - it probably is.
I just wouldn’t use Slack, personally, because I don’t like walled (proprietary) gardens much.
My point is: why introduce yet another community platform?
Reminds be a bit of that xkcd comic about the competing standards…
Instead, I think Yii ought to make the forum so good that people would want to use it, a lot!
Discourse, or maybe NodeBB, would be a good upgrade I think, and it will have modern, Slack/Gitter like features.
And the Yii project would not have to put all of the eggs into a proprietary third-party basket 
Edit:
Yii already have a Facebook group, a Gitter room, IRC, StackOverflow presence .
and probably something that I overlooked.
It really shouldn’t have another thing.
A new and irresistible forum to replace the one we currently have would be nice! 
Preferably a forum of which we can say:
A bit like Slack, but better.
Similar to StackOverlow, only better.
Akin to a Facebook group, although more sassy!