>>38281613He moreso meant in the sense that banning a Pokémon because of a move opens up Pandora's Box in terms of banning Pokémon: Why not suspect test Reshiram, but banning it from using Earth Power?
As you've said, It is true that Aegislash is the only user of King's Shield, and the complex ban of the move wouldn't hinder anyone, but I don't think the problem lies so much in putting up the rules themselves, although I think some Showdown admins have stated it can be a hassle to make, but it's like the whole " Goku in Smash " case.
