But we found that in the end - a normal wiki was actually more useful. I think this was mainly because of how our organization works. We're all physically on the same campus, at most a colleague is 2 minutes walking away. Hence we tend to often have in-person communication, or first via Slack, after which a colleague walks over to explain it anyway..
Using a wiki, the person knowledgeable about the domain can just create a document to refer people to which is quite natural. What is not natural for this person is to create a question and then answer it himself. (I know you could use SO as a wiki but than you're losing some of the utility anyway).
I'm not saying that it can't work, but a lot depends on company culture / structure, at least I think so. So if you're in a situation where people often post questions on slack and people respond through slack, maybe SO is a good alternative.
(Oh also, on another note, during our test people would post question on SO and no one would answer because no one is looking, or people think "someone else will answer" and don't bother to check up later. Via slack, at least it's more visible and scoped to the correct channel with the correct people for the domain).
YMMV ofc :)