This is more or less what the sandbox mechanism on macOS does today. Unless a sandboxed app was given special entitlements from Apple, it can only access files

This is more or less what the sandbox mechanism on macOS does today. Unless a sandboxed app was given special entitlements from Apple, it can only access files that were brought into the sandbox at some point (for instance by selecting it in an 'open file' dialog, aka 'powerbox').