The scanning wasn’t to be done by the OS, but by the Photos app uploader, and only if you had iCloud Photo Library turned on. Lots of apps already do telemetry

The scanning wasn’t to be done by the OS, but by the Photos app uploader, and only if you had iCloud Photo Library turned on. Lots of apps already do telemetry and photo scanning that goes beyond this, as well as nearly every major cloud provider (though Apple seemingly doesn’t scan iCloud photo libraries without a warrant unlike Google and Facebook). Like a lot of apps, you don’t need to use Photos.app. They didn’t sneak it in without explaining how it worked.