Mirror-Shield Hoplite

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• 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 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
  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.
• 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
  If a creature other than the creature with backup ends up as the target of both the backup ability and the copy, it will end up with two +1/+1 counters and it will gain the non-backup abilities twice. In some cases, the duplicate abilities will be redundant (two instances of flying, for example). However, if it picks up two triggered abilities, for example, those abilities trigger independently.
• WWizards of the Coast2023-04-14
  Official Ruling
  The source of the copy is the same as the source of the original backup ability. If the creature that ends up as the target of the copy is that original creature with backup, it will get a +1/+1 counter, but it won’t gain any abilities. If it’s a creature other than that original creature, it will get a +1/+1 counter and gain abilities.
• 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.

Rulings via Scryfall