According to the 24w33a changelog:

Any item or block which can be crafted must inherit the highest rarity of its crafting ingredients.

Crafting a shield and an ominous banner together does not result in an uncommon shield, even though the ominous banner is uncommon.

How to reproduce:

1. Give yourself a shield and an ominous banner.
2. Either open the survival inventory or place down a crafting table and open its GUI.
3. Place the shield and the ominous banner in the crafting slots.
4. Look at the crafting output.

Expected result:

The decorated shield resulting from the crafting recipe would be uncommon, since the highest rarity of its ingredients is uncommon.

Observed result:

The decorated shield resulting from the crafting recipe is common, despite the ominous banner used as an ingredient.