Every monster has a specific AC in its stat block. The rules don’t explicitly say anything like “subtract 2 from AC if the creature doesn’t use a shield”, do they? Does that mean a monster always has a fixed AC number regardless of the situation, even when it doffs the shield?
For example, the Goblin statblock says:
Armor Class 15 (leather armor, shield)
A goblin attacks with a shortbow. Since a shortbow is two-handed, the goblin can’t wield a shield at the same time. Should a goblin archer’s AC be 13 without a shield, or is it still 15?