I have the feeling something changed in the implementation of the GAME-code askpromote command that broke it.

Not exactly broke. More that your expectations about how it works are not up-to-date.

Above the Pawn there is a message that says it declines promotion. I don't recall having seen that before.

This appears only where there is an option to not promote.

After you make the choice and submit it through the button, it seems the original move is gone. The askpromote screen is supposed to append the choice to the move you had so far, in the form "; X-dest" (where X is the label of the chosen piece). But here it seems to replace the move by that, so that this now becomes the only move primitive. Which is of course invalid.

I am not aware that it replaces anything. Looking at the code for askpromote, which you can view yourself here, I only see where it keeps the original move without appending anything additional to it. This is for when the piece does not promote. Adding something like P-dest to the end of the move is an ugly kludge that I figured out how to do without. If you take a look at the Pawn subroutines in the fairychess include file, you will find code that handles optional Pawn promotion without the need to append a fake promotion to the end of the move. I believe the critical code looks like this:

if onboard where #to 0 #pzs: // Not yet in promotion zone
  if != space #to $moved:
    set name alias const alias $moved;
    die "You may not promote a" #name "until it reaches the promotion zone.";
  endif;
elseif onboard where #to 0 1: // Not yet on last rank
  if == White_Pawn const alias space #to and count var wprom:
    if not $answered and == mln $maxmln:
      push wprom space #to;
      askpromote #wprom;
  endif;
elseif ...

The first if-block is executed if it is not yet in the promotion zone. So, each subsequent elseif-block can be executed only if it is in the promotion zone. The second elseif-block executes if it is not yet on the last rank. So, if it does execute, the Pawn is in the promotion zone but not yet on the last rank. It then checks whether there is anything to promote to. If there is, then it checks two more things. And these are the important things to pay attention to. It checks whether this is the last move by comparing mln with $maxmln, and it checks whether askpromote has already been used on this move by checking the $answered flag. As long as it is the last move, and askpromote has not already been used on this move, it adds the Pawn to the promotion options and calls askpromote. Because it checks these two things, it does not have to check whether the move already contains a promotion, and that allows a player to decline a promotion without adding a fake promotion (or a skip or pass) to the move.