I’m not sure I subscribe to the idea that’s heat damage. That looks like friction damage from the plate frame rubbing on the bumper over time, and/or someone getting real aggressive trying to get bugs and grime out of
there.
It’s been a long time since I took mine off, but as I recall it wouldn’t even make sense to apply heat there. The painted part of the plate holder comes off with just the aforementioned bolts and screws behind the plate itself, leaving the smaller black plastic mounting point that is affixed with the tape. That’s where you’d apply some heat if necessary.
Even if it is heat damage, I’ll stand by my statement that you’d have to try real hard to damage the bumper like that. Like basically being clueless or negligent.