I have tried so hard to get people to do this, but it seems like a lot of my coworkers are honestly scared of "public" channels, and by "public" I mean a channel with only 15 or so people on it. They insist on DM'ing most things, and some just refuse to use chat at all; those will only communicate via email.
I'm active on a programming language Slack space, what sometimes happens is that people are DMed directly with a generic / open question that dozens of others could also answer.
I know for a fact that anonymity can be used to encourage diversity of thought and honesty on public channels - this is the feedback I get from customers of a service I run (https://AnonymityBot.com).
Only if that room enables people to only hear shouts that include their name, where they can pause hearing shouts, where it's acceptable to ignore shouts if you're busy, and where there's someone recording the shouts in case someone has a similar problem later.
In fact, in almost all the ways that matter it's entirely different to shouting in a room.
I would prefer to answer dumb questions all day long than have people on my team grind to a halt for a day because they're scared of asking for help.
They're just doing it via DM instead of in the 'public' chat.
Slack is literally telling folks: 'You are holding it wrong'.
In reality their product isn't that great for q&a or knowledge sharing. Horrible search, bad notification, high noise to low signal.
And quite often, you get better and faster response when you ask a person instead of forum.