I wouldn't call the shrapnel a retcon since we don't really know what happened to BB during this time period. Kojima can add all the random crap he wants as long as BB returns to the same state he was in MG. If the shrapnel is still there in a remake, then it would definitely be a retcon.
Technically, the robot arm isn't a retcon either, but I'm going to consider it one since it was never implied that BB had a prosthetic arm in MG.
Technically, the robot arm isn't a retcon either, but I'm going to consider it one since it was never implied that BB had a prosthetic arm in MG.