[sellyme]

> The game has since been stable for about 500.

There were a few patches to chess in the mid to late 20th century that disallowed promoting a pawn to an enemy piece (this was useful for forcing a smothered mate), and to prevent vertically castling to a pawn that had been promoted into a rook on the 8th rank (notated as 0-0-0-0-0-0).

Not exactly fundamental changes, but still amusing that they needed to be made after so many centuries.