I think that the advantage in using some kind of instant-message system like Discord/Slack is that you might end up getting a faster response, which is quite helpful when I'm feeling impatient and want my problem to be solved faster. On the other hand, the questions aren't as easily searchable and archived for future people to find.

And we are back to a "decentralised" web, although it's only decentralised between a few servers in Discord's and Slack's basements.

It is a problem for ephemeral vs evergreen.

Chats are a horrible place for evergreen knowledge. Forums are pretty good though.

It’s only a slight difference but a big one.