Summary:
As of https://www.minecraft.net/article/minecraft-snapshot-21w13a leather horse armor prevents horses from freezing in powder snow. As of 24w05a, where the way that horse armor is stored was changed, this no longer works.

Notes:

• Relates to MC-268934, MC-270539, MC-270301. (All of these issues were introduced in 24w05a with the change that horse armor is stored in body_armor_item instead of ArmorItems[2].)
• Horses from before 24w05a that were wearing leather horse armor will be protected from freezing even if you unequip the horse armor from the horse, since the leather horse armor remains stored in ArmorItems[2] after upgrading.

Steps to reproduce:

1. Tame a horse
2. Equip it with a saddle and leather horse armor
3. Place powder snow
4. Ride the horse into powder snow

Observed results:
The horse freezes despite wearing leather horse armor.

Expected results:
The horse should not freeze, since it is wearing leather horse armor.