I can think of a very obvious one, identify and refuse to send messages that the client app decides are child porn. No intrusion.

Or perhaps add a counter to the account when it's detected. Minimal intrusion, single flag defining the message.

You don't need to mitm things to implement _some_ mitigations.

Before the inevitable - a method that is not 100% reliable in stopping something is not useless. Otherwise we may as well make it as easy as possible to share child porn because it wouldn't make a difference.