> The other is detection of images that may contain nudity, whether sent or received, when the owner/admin/parent enables the feature. It is relatively uncontroversial [...]
That's a new version. The one that as announced the same time as client side scanning to block uploading CSAM to iCloud worked like this.
1. It could be enabled on a child's device by the parents. It was not on be default.
2. If the child received a sexual image (not necessarily just CSAM...if an adult sends a dick pic to a child that is not CSAM but would have been flagged) the image is blocked, the child is notified, told their parents are worried the image may harm them, and asked if they still want to see it.
3. If the child says no, they do not want to see it, that is the end of the matter.
4. If the child says that they do want to see it and they are at least 13 they are shown the image and that is the end of the matter.
5. If the child says that they do want to see it and they are under 13, they are again told that they parents are concerned, and that if they view it their parents will be notified, and asked if they still want to view it.
6. If they say no that is the end of the matter.
7. If they say yes they see it but the parents also are notified and will be able to see it.
This should have been pretty uncontroversial, but there were objections on the grounds that if someone say sends their dick pic to your under 13 child and the child goes all the way through to step 7 and decides to view it, that is a violation of the sender's privacy because that message was only intended for the child.
One of the more recent changes is that anyone can now enable a version of this functionality, even if they aren't set up with a child account. (It's under Settings > Messages > Sensitive Content Warning.) In this mode, it behaves similarly to the over-13 mode described above; no one is notified, whether the user chooses to view the image or not.
This mode is probably best understood as "if someone texts me a dick pic, please blur it". I don't think there's any reasonable objection to this.
I have a hard time imagining any significant number of people objecting based on the sender's privacy. An adult sending explicit images to a child is a crime in most jurisdictions. A child doing it with a recipient under 13 is a concerning behavior the parents need to address.
Notifying the parents even requires that the child acknowledge that's what's going to happen.
Presumably they were worried about misclassified images, or images sent by other kids.