Mystic Subdual

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2020-04-17
  Official Ruling
  Mutating the enchanted creature won’t cause it to gain abilities. This is because mutate modifies the creature’s base characteristics, while Mystic Subdual applies to the resulting creature.
• WWizards of the Coast2020-04-17
  Official Ruling
  If a creature has power and toughness written as */* with an ability that defines its power and toughness, its base power and toughness become 0/0 when it loses all abilities. Mystic Subdual then makes it a -2/0. If its power and toughness are written as */*+1, it’s -2/1, and so on.
• WWizards of the Coast2020-04-17
  Official Ruling
  If the affected creature gains an ability after Mystic Subdual becomes attached to it, it will keep that ability.

Rulings via Scryfall