> So for example, I want to schedule a doctor’s appointment on my personal and have my other calendars mirror that time block, but scrub the details/make it generic. Super specific idea obviously

Actually not that specific at all! I know a lot of people have at least the work/personal schedule split, and personally I have the additional side project one the same as you, which I bet is pretty common among people putting on this very website.

I also know a lot of people have other arrangements they'd like. Partially out trans people jump to mind. And I'd like my personal contacts to know what my side project life is up to but not vice versa, which is probably not standard but I doubt I'm alone.

So I think customizable ways to separate your life and who is able to see it is not really a niche feature at all, and just a commonly missing one.