This description of the macOS model doesn't really apply so I'm not sure if I'm misunderstanding you or you're misunderstanding the model.
> Another approach is to police everything behind rules (the way selinux or others do), which is even better in theory. In practice, you waste a ton of time bending those policies to your specific needs. A typical user won't take that.
While SELinux could probably provide this kind of data protection on Linux, the method of technical enforcement is only one part. There's a lot of UI involved to get right, and that will require far more effort.
> Then there is the flatpak+portal isolation model, which is probably the most pragmatic, but not without its own compromises and limitations.
That model doesn't really apply here. Flatpak et al allow applications to self confine in order to protect the other things the user is doing. What I'm talking about is for an app to have some protections of its own data from the other things the user is doing. I'm not talking about sandboxing, this data protection.