Arrester's Zeal
Instant
Target creature gets +2/+2 until end of turn.
Addendum — If you cast this spell during your main phase, that creature gains flying until end of turn.
Law is the light of civilization.
Izzy
The List (PLST)
• #RNA-4Rulings
Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
(wotc • 2024-01-12)
If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.
(wotc • 2024-01-12)
Rulings
Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
(wotc • 2024-01-12)
If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.
(wotc • 2024-01-12)