This was abused by several apps to allow locating and identifying users illicitly. One app, say, WeChat, would ask for broad permissions for location and unique identifiers and then leave unique tracking identifiers in a shared file location. Than another app, say, a mobile game, could pick up on those shared identifiers and link you back.

Why this would be a useful benefit worth setting up in an SDK across multiple companies and apps, I'll leave to your own imagination.

This would be solved if they just ported over the revokable permissions that LineageOS implements. You get real security and the app is none the wiser.

It's not like the switch to scoped storage would mitigate much of it. You can still do all kinds of nasty things when you, for example, have access to the photo library part of the MediaStore content provider. Like adding your tracking ID to the metadata of a picture that other app would then look for. Or, you could just create a "photo" and write your arbitrary data in place of the file contents. Users won't ever notice there's a broken image at the end of their camera roll.

Will this change make it look more suspicious when apps request access to photos instead of storage? Yes, sure. Will most users care about it and deny access to apps that aren't supposed to have it? Of course not.

Why would you even break the photo? Steganography goes back hundreds of years before computers. The only challenge is too make the user keep some of the watermarked pictures (the OS can keep you from writing to files created by other apps. They are sacrificing far too much for a fight they cannot win.

I wonder how this will impact my externally stored music library, which is accessed by two open source apps (Vanilla Music and Alarm Klock) but not managed by those apps or tied to their lifetimes.