They also use on-device machine learning to detect pictures (and videos?) containing any kind of 'undesirable' content.

"Messages uses on-device machine learning to analyze image attachments and determine if a photo is sexually explicit."

Furthermore: "This program is ambitious, and protecting children is an important responsibility. These efforts will evolve and expand over time."

Per: https://www.apple.com/child-safety/

The road to hell is paved with the best of intentions.