Maybe that would be obvious to the average Apple user, but as a non-Apple user, it would never occur to me that I'd have to explicitly de-register my phone number with Apple after activating a new phone.

That seems okay? It's only applicable to the Apple user to begin with.

heh, fair point. I was mainly thinking about somebody dipping a toe in or trying out an iphone for a little bit before switching back

There was a time after you still needed to have the iPhone to do it, though. And I'm not sure how many are actually aware you have to do this, just getting messages for them sent into the void.