If the Ninja is destroyed or otherwise removed from the field by a card effect, “Ninjitsu Art of Decoy” remains on the field meaninglessly.
If the Ninja is removed from the field temporarily, or flipped face-down, “Ninjitsu Art of Decoy”’s effect is no longer applied.
If “Ninjitsu Art of Decoy” is active, then “Royal Decree” is activated and negates “Ninjitsu Art of Decoy”’s effect, then “Royal Decree” is later destroyed, “Ninjitsu Art of Decoy”’s effect is not re-applied.