This I think got removed somewhere around v2, but I use these features all the time on other services (Slack, Twitter, Facebook, etc) and we have no equivalent. I think you can maybe do something like this today with locking down a Policy on a room, it's cumbersome and not super user friendly like other services.
As a baseline example, the designer Slack chat I'm in sends about 10k messages a week with 30-50% of them direct user to user.