Would apple/google be able to comb through that data though? WhatsApp backups are not e2e encrypted, but they're still encrypted with a key generated by Facebook the moment you login to your account, and probably refreshed periodically. Technically, Facebook would be the only one capable of turning the encrypted backup into plaintext, so if google wanted to do something with your messages backup it would have to knock at Facebook's door first.

This kind of cooperation is definitely illegal in this moment and I'm nut sure future legislation will require services to set up persistent encryption keys exchange among services so that a backup file that is supposed to be encrypted on a cloud storage can be continually scanned for harmful/unlawful content by another service. If the EU or any government wanted to set up such kind of monitoring, they will require these services to adapt in a way that these kind of gimmicks with backups and encryption keys are not needed