I'm working on something like this for Android. My goal is to build a system where plugins can selectively block parts of apps based on a set of rules. I can't promise that this will end up making it into the final version because the Android documentation states that the accessibility APIs may only be used for accessibility tools, but it's what all of the other app blockers on android do so fingers crossed they let me do it as well.

This would also be possible for desktop, since I'm using browser plugins to block websites on Desktop at the moment. I don't think that this is possible on iOS, but there may be APIs that could enable something like it that I'm not aware of. As much as I like Apple's commitment to privacy, the way that they've locked down a lot of their APIs has been a real thorn in my side for enabling some of these more advanced use cases.