Single Comment

Even if en passant and castling moves were fully written out, so that automation was not required for these moves, it would be a mistake to check only the last move, because the legality of these moves depends not only on the board position but also on the past history of the moves. Checking all prior moves each turn allows for the bookkeeping that must be done to determine if an en passant move or castling move will be legal. Besides, checking only the last move won't save much time. Checking a series of moves doesn't take very long, even for long games. Compared to checking for checkmate and stalemate, which require checking out alternate lines of play, checking the legality of a move doesn't take very long at all.