Note that:

1) its only scanned on upload to iCloud, so if you don't upload then its not scanned

2) (per another article, https://techcrunch.com/2021/08/05/apple-icloud-photos-scanni...): Most cloud services — Dropbox, Google, and Microsoft to name a few — already scan user files for content that might violate their terms of service or be potentially illegal, like CSAM.

So you really can't opt out unless you avoid all cloud photos