Okay. For now. But it’s a trivial change to begin reading my cache.

The technical details and current policies don’t really matter. The bigger picture is terrifying.

Sex crime accusations, even if nobody is convicted, completely ruins lives. I don’t want a robot on my phone doing that.

I don’t think it’s as trivial as you suggest, but yeah they could do that – they always could have made changes to flag crime or give access to criminal’s devices but they still refuse to do so.

The barrier for being accused by someone who matters is quite high, you’d need to breach a certain threshold of material. That material is then manually reviewed by a human and only then if it appears to be CSAM do they refer your details to the police. The police would then presumably also check the material is bad before arresting you and ceasing all your devices.

for 4chan maybe that's true but I'm not sure what about some public whatsapp group? I have been part of few public hiking/travelling group and even though I have most of them muted (to avoid distraction) all pictures end up in my Photos 'Recent' Album.

If you see something, say something, and report the person sending that stuff.

You can turn that off in Settings -> Chats -> Save to Camera Roll

But what if you don’t realize you can do this or you forget to? This should be off by default if you ask me.