I think, codewise, zombies (all monsters) are represented much more simplistically than npc’s. They don’t have separate pools for their limbs afaik, just one overall health pool. So there’s nothing to target. Special attacks that inflict status effects simulating limb damage…now that’s doable.
Limb targeting would be handy for NPC fights. I’d love to target the hands to disarm them, target legs to slow them down so I can pick them apart, etc…