The legal and liability protection these messaging services get from E2EE is far too big to break it.

Besides I get the feeling we're so cooked these days from marketing that when I get freaked out that an advert is what I was thinking about. It's probably because they made me think about it.

Or maybe I need to update my meds?

Assuming there's no code review or audit, I suppose.

How would you hide that? Unless you’re assuming nobody ever has to try and fix bugs or audit code to find it, and there’s some kind of closed off area of code that nobody thinks is suspicious. Or you maintain a complete second set of the app core libs that a few clandestine folks can access, and then hope nobody notices that the binaries don’t line up and crash logs are happening in obscured places.

I would be surprised if the code was hidden from other people engineers.