Bristlepack Sentry

• WOTC Official Reminder Textofficial rules
  Defender — This creature can't attack.

• WWizards of the Coast2024-04-12
  Official Ruling
  If you raise Bristlepack Sentry’s power to 4 or greater, it fulfills its own condition and it can attack as though it didn’t have defender.
• WWizards of the Coast2024-04-12
  Official Ruling
  Once Bristlepack Sentry has legally attacked, causing its last ability to not apply by removing other creatures from the battlefield or reducing the power of one or more creatures won’t cause Bristlepack Sentry to stop attacking.

Rulings via Scryfall