Sure, but they'll be banned by Apple / Android ToSes. But meh, that shouldn't stop everybody. And it would be fun to watch the back and forth between ad tech and ad blockers.
True. Do carriers try to add stuff like this to their ToS? I usually ignore them and root my phones anyway, so I'm not current on which part of the "value chain" is offering a ToS.
Google may.
No other variants of Android do, AFAIK.