Watcher for Tomorrow

• WOTC Official Reminder Textofficial rules
  Hideaway — When this permanent enters the battlefield, look at the top N cards of your library. Exile one face down and put the rest on the bottom in any order.

• WWizards of the Coast2019-06-14
  Official Ruling
  If Watcher for Tomorrow leaves the battlefield before its triggered ability from hideaway resolves, its leaves-the-battlefield ability resolves and does nothing. Then its enters-the-battlefield ability resolves and you exile a card with no way to return it to your hand.
• WWizards of the Coast2019-06-14
  Official Ruling
  You don't reveal the exiled card when you put it into its owner's hand.
• WWizards of the Coast2022-04-29
  Official Ruling
  Any player who has controlled a permanent with a hideaway ability since a card was exiled with it may look at that card.
• WWizards of the Coast2022-04-29
  Official Ruling
  Hideaway now causes you to put the rest of the cards on the bottom of your library in a random order instead of any order.
• WWizards of the Coast2022-04-29
  Official Ruling
  "Hideaway N" means "When this permanent enters the battlefield, look at the top N cards of your library. Exile one of them face down and put the rest on the bottom of your library in a random order. The exiled card gains 'The player who controls the permanent that exiled this card may look at this card in the exile zone.'"
• WWizards of the Coast2022-04-29
  Official Ruling
  Previously, permanents with hideaway entered the battlefield tapped. This ability has been removed from the definition of hideaway. Older cards have received errata to have an additional paragraph that reads "[This permanent] enters the battlefield tapped," and they now have hideaway 4.

Rulings via Scryfall