I turned xkcd’s offside chess into a playable variant. A checking piece is offside if it has advanced beyond every other friendly piece; you can apply the rule to checkmates only or to every check.

Running the rule against Lichess games, 63% of checkmates and 57% of checks would be offside. This makes attacks develop much more slowly. The “any check” version produces especially strange endgames: kings are nearly immune to checks and can blockade with impunity.

You can play locally, against a modified Fairy-Stockfish, or watch the engine play itself.