I agree with Damian, but was unsure of a better name for it. I mostly created the tag to be able to fix some cases I knew were first-party and get rid of the implicated third-party edit tags implied after this was changed.
I agree that in the majority of cases it's the version with the background that is composited with this isolated version as a component rather than the background being directly removed. Not sure if "isolated" is very intuitive though since that would more likely make me think of a lone character on a desert island or something like that. The other suggestions also have multiple meanings, which is somewhat problematic.
As a side note, this tag is going to be a royal pain to populate. I tried to do it for posts I didn't have first-hand knowledge of, and there isn't a great way to query for them at the moment that I could think of.