Subtlety

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

• WWizards of the Coast2021-06-18
  Official Ruling
  To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as an evoke cost), add any cost increases, then apply any cost reductions. The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.
• WWizards of the Coast2021-06-18
  Official Ruling
  The owner of the spell is the one who chooses whether it goes on the top or bottom of their library. All players will know this information.
• WWizards of the Coast2021-06-18
  Official Ruling
  Subtlety's triggered ability targets a spell on the stack. It can't target creatures or planeswalkers on the battlefield.
• WWizards of the Coast2021-06-18
  Official Ruling
  A spell that's put into its owner's library doesn't resolve, but it isn't countered. This will work on spells that say they can't be countered.
• WWizards of the Coast2021-06-18
  Official Ruling
  If you pay the evoke cost, you can have the creature's own triggered ability resolve before the evoke triggered ability. You can cast spells after that ability resolves but before you have to sacrifice the creature.

Rulings via Scryfall

• WO
  Wizards of the Coast2024-06-03

  Subtlety was banned in historic on June 3, 2024. We have been cautious of cards that can be cast for free with alternative payments as we believe they are too fast for this format. So, it should not be surprising that we are banning the Evoke Elementals and Flare cycle for Modern Horizons 3. Source: https://magic.wizards.com/en/news/mtg-arena/mtg-arena-announcements-june-3-2024