You can set up a non federated matrix server where you can then control who gets access to what via invites to rooms and communities. So, you can kinda do it, a

You can set up a non federated matrix server where you can then control who gets access to what via invites to rooms and communities. So, you can kinda do it, altough it is probably not the best service for a 1:1 translation of a slack setup. But if you are willing to somewhat alter it, I am sure it'd end up with a overall better setup.