I'm not sure it is an unsolved problem. Most people with this "issue" already have a solution: multiple groups within a platform, multiple platforms or multiple

I'm not sure it is an unsolved problem. Most people with this "issue" already have a solution: multiple groups within a platform, multiple platforms or multiple accounts. For each group I need to communicate with, I have existing communication methods. That's one of the ways that I know that they're a distinct group. I don't worry about my students seeing all my Whatsapp messages to my friends because I don't communicate with students over Whatsapp - it's not a good tool for that, and I can use pre-existing channels of communication. If I did want to communicate with students over Whatsapp (which is prohibited by lots of organisations because it's rarely a good idea), I would put them in a dedicated group and message that directly. Basically, I think you only encounter this problem of not being able to share information with only specific people/groups if you've previously spent a bunch of time cross-pollinating the distinct groups. If you're not using Teams to recommend your Discord so that you can promote your niche art Tumblr, you'll find that people put themselves into those 'sharing rings' automatically.