I know that mobs aren't programmed to use their offhand but some of them have an offhand slot so it is possible for them to equip shields, totems, arrows, etc. with the use of commands. However, the items on their offhand show a visual glitch where the item they're holding is positioned on their mouth. Yes, I understand that you will never see mobs hold something in their offhand (except Piglins) in survival but this visual bug needs some fixing since it makes the game look broken in terms of technical stuff like this. Hoping to see a Vindicator hold a shield the right way when this is fixed