Boon-Bringer Valkyrie — ruling 1

If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.

Boon-Bringer Valkyrie — ruling 2

If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.

Boon-Bringer Valkyrie — ruling 3

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.

Boon-Bringer Valkyrie — ruling 4

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.

Boon-Bringer Valkyrie — ruling 5

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.

Boon-Bringer Valkyrie : règles, fonctionnement et interactions (MTG)

Boon-Bringer Valkyrie is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Backup 1 (When this creature enters, put a +1/+1 counter on target creature. If that's another creature, it gains the following abilities until end of turn.) Flying, first strike, lifelink

How it works : Boon-Bringer Valkyrie

Original English text — Wizards of the Coast / Scryfall

#1April 14, 2023· Wizards of the Coast
If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.
No other card is named in this ruling.
#2April 14, 2023· Wizards of the Coast
If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.
No other card is named in this ruling.
#3April 14, 2023· Wizards of the Coast
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.
No other card is named in this ruling.
#4April 14, 2023· Wizards of the Coast
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.
No other card is named in this ruling.
#5April 14, 2023· Wizards of the Coast
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.
No other card is named in this ruling.