I had Dark Leo and Shredder on the field and the opponent gave it -3/-3 (I think it was Karai's Technique). My creature stayed on the field as a -2/0 although it was also in the graveyard. I couldn't attack with the creature, its trigger didnt work, and I also played a second one and didnt trigger legend rule. But the creature that the game knew was removed was still visually on the board. Im not sure if it was the same for the opponent.