“What happens on your iPhone stays on your iPhone”
Seems like this is still true. CSAM checking only happens when you choose to upload your photos to iCloud Photo Library.
Apple has been using some dark patterns to force people to use iCloud. Telling them they haven’t finished setting up their phone. It notifies you until you sign up for the free tier, and then when that’s filled up it pesters you again
That’s true, and a fair criticism, but it’s quite different from saying people don’t have a choice.