We acknowledge the customer demand for Private Channels and have been hard at work on it. Private Channels is currently in preview with a select group of customers. We expect to release the feature to the public later this Fall, and will update here when we are starting to roll out to all customers. We appreciate all the feedback you have provided to us on this forum and thank you for your patience.
I'd like to add that for my organization (and many others), this isn't simply a matter of convenience. It's a dealbreaker that will force us to another solution, because we have a security requirement to segregate sensitive content. I need ALL users to have access to about 80% of the material that is shared, but I can only allow a subset of those users to access the other 20%. It has nothing to do with whether the content is interesting or relevant. They're just plain not allowed to see it, and they shouldn't be part of any conversations around that content.
The workaround is to maintain at least two if not three separate teams for each collaboration effort. This simply doesn't scale once you get beyond the first handful of projects. I am looking at a use case that is repeated 25+ times per year, usually with a unique group of participants.
Right now it's much easier for me to just use Slack for conversations and manage the content separately on SharePoint as if we were stuck in 2007.
This feature request is still on the backlog. I’m waiting to hear more that I can share.