In the just-released annexes of the 90th Congress it turns out that in April 2019 the Rules Commission discussed a number of possible changes to the Laws. I expect that these are only at the earliest in-principle approval stage pending consultation, drafting etc. See

One of them is:

Rules Commission suggests changing Article 5.1.1 to:
Article 5: The completion of the game
5.1.1 The game is won by the player who has checkmated his opponent’s
king. This immediately ends the game, provided that the move his last
& previous moves producing the checkmate position were in
accordance with Article 3 and Articles 4.2 – 4.7.

For a normal game, this would mean that if A completes an illegal move, B makes a legal reply or completes an illegal reply, and A then delivers checkmate with a legal move, the checkmate would not stand and the position would be wound back to A's previous move.

Without further wording changes it would be problematic for blitz and rapid where the illegal move cannot be wound back! If it was implemented such that the previous illegal move in rapid and blitz could be wound back for this special case, it would create a situation where A could benefit by playing any move that wasn't mate (eg forcing mate in 2 instead of playing mate in 1) to ensure the position couldn't be wound back.

I'm not sure how common a problem an illegal move followed directly by mate is. A more common problem in my experience is one side plays an illegal move and the opponent immediately resigns.

(Not sure how much interest there might be in the other proposed changes, or what their status is in terms of who has proposed them and what Rules thinks of the changes, but feel free to comment on them here. The one to is silly as excessive draw offers are already prohibited and trying to put a number on it just gives excessive draw offerers a pretext for trying to get away with it.)