They only need to get caught doing it once to have a gigantic lawsuit on their hands. They could possibly pull it off very selectively, but doing it in a dragnet fashion seems incredibly risky.

My suspicion is that things like URL previews, or when the app calls a handler to open a link (YouTube, browser), are still monitored.

So e.g. opening a link to an Amazon product that a friend sent makes you a target for ads in that category.

It's a suspicion, they'd probably argue their EULA allows this. The typical "we have to monitor links for dangerous content" is always the standard bullshit.