e621 is a preservation site as far as I'm aware, so it seems counterintuitive to delete something just because the artist got banned.
It just feels like a weird separating art from the artist type deal, you shouldn't burn a history book just because it mentioned some horrible person, It's still a part of history.
I could understand if the artist themselves requested removal, and maybe that is what happens most of the time, but I feel like that should be made a bit more clear if that is the case rather than "artist/character got shown the door".