iMessage does not do this in any release, that plan was abandoned due to legitimate privacy concerns. The feature that is implemented is a parental control that

iMessage does not do this in any release, that plan was abandoned due to legitimate privacy concerns. The feature that is implemented is a parental control that can be enabled on the devices of minors, that blurs any detected nude images with a warning 'do you want to see this'. It's not possible to detect CSAM without breaking the security model of the system for everyone, if the system can make any kind of report of detected content to an outside entity. If the system scans messages client side and then sends anything to do with the message to authorities, that is breaking the end to end encryption. This is either done by 'AI', which means huge numbers of false positives that are actually legal, private and probably intimate get sent to authorities (hence breaking the end-to-end security model for that legal content), or by a secret, un-auditable collection of hashes - which are still fuzzy matched, so there would definitely be false positives (again, breaking the security model for legal content), but potentially also Governments could start to add other, legal material to the database, such as certain political material to detect dissidents. Since it's un-auditable, there is no way to know whether it's actually just illegal material in the database. So whether it's AI or PhotoDNA or whatever, there's no way to do it that doesn't break the security model and cause more danger to innocent people, including destroying the privacy of children as well as adult, law-abiding citizens.