I just discovered twist. Looks like something worth keeping an eye on!
Like Discourse, threads (e.g. topics) are first class citizens and they have decided against a presence indicator.
But like Slack, they have the channels you have chosen to join shown persistently on the left, rather than only having all categories under a dropdown or on a separate page (related topics here and here).
I’m also curious to hear other first hand impressions if anyone else here has played around with it.