Apple's Photos app can search photos based on their contents (e.g. try typing "cat" into the Photos search box). It can also identify individual faces, and group photos based on who's in them.

All of this is local-only. (Which is why it has to run an expensive indexing process locally.)

Cool tech, but so frustrating if it can't be toggled as an option. Reading this it's clear that I would be even more incompatible with macos now than when I left it years ago, just the lack of control.

It can be.

I have my photos on Synology NAS, and it does the same thing - except it manages doing it much more efficiently, while using a much weaker Atom CPU.

photoanalysisd was responsible for making my Mac feel slow at least 80% of the time I started to notice it chug. Apple should really limit the processes CPU time so that it doesn't randomly spin up my fans.

The CSAM thing "came to light"? They announced it publicly and proudly, before it was even implemented! There's no reason to think they are hiding anything.

The code for this was found in iOS 14.3, just in an inactivated state.

It boils down to using Linux if you want privacy.