There have been plenty of cases when signature moves were made no longer exclusive to its user, such as Victini's V-Create on Rayquaza or Manaphy's Heart Swap on Magearna, so defining what counts as a signature move is a bit tricky. As these examples still have a very small distribution, you could argue that they're still signature moves even if they're not exclusive. Mystical Fire on the other hand went from being the signature move of Delphox in XY, with a later limited distribution to Mismagius in ORAS, to becoming a TM in SS and having a pretty good distribution. Should a command like this take in to account what generation you're searching in or just show when a user had an exclusive move?