Doomskar Warrior

• WWizards of the Coast2023-04-14
  Official Ruling
  If a permanent enters the battlefield as a copy of a card with a backup ability or a token is created that is a copy of that card, the order of the printed abilities is maintained.
• WWizards of the Coast2023-04-14
  Official Ruling
  If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.
• WWizards of the Coast2023-04-14
  Official Ruling
  In some unusual situations (mostly involving damage redirection), Doomskar Warrior may deal combat damage to multiple players and/or battles at the same time. If this happens, its ability will trigger for each of those players and/or battles. Each ability will resolve separately, allowing you to look at a number of cards equal to the damage dealt to that specific player or battle.
• WWizards of the Coast2023-04-14
  Official Ruling
  The abilities that backup grants to the target creature are determined only once, at the time the ability triggers. They won’t change if the permanent with backup loses any abilities before the backup ability resolves.
• WWizards of the Coast2023-04-14
  Official Ruling
  If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.
• WWizards of the Coast2023-04-14
  Official Ruling
  Backup confers only abilities that are actually printed below it. Any abilities that are gained by the permanent are ignored, including abilities gained due to a resolving spell or ability or copy effects.

Rulings via Scryfall