Soaring Thought-Thief

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

• WWizards of the Coast2020-09-25
  Official Ruling
  An ability that offers a bonus if an opponent has eight or more cards in their graveyard won’t provide additional benefits if more than one opponent has eight or more cards in their graveyard. There are also no additional benefits no matter how many cards are in an opponent’s graveyard, as long as there are at least eight.
• WWizards of the Coast2020-09-25
  Official Ruling
  In a multiplayer game, once an opponent leaves the game, they won’t be considered for such an effect, no matter how many cards they had in their graveyard before leaving the game.
• WWizards of the Coast2020-09-25
  Official Ruling
  Soaring Thought-Thief’s third ability applies to itself as well as other Rogues you control.
• WWizards of the Coast2020-09-25
  Official Ruling
  Soaring Thought-Thief’s fourth ability causes each opponent to mill two cards, not just those being attacked by Rogues you control.

Rulings via Scryfall