I have a shield on a powerful 10th level NPC that is magical and has an Intelligence of 12. I was laying in ground work for future plots and had my 3rd level players meet them for a moment. Someone cast heat metal on the NPC's shield and the NPC did failed the CON save. I made the call that it effected the NPC and the shield was dropped. Though I have been thinking that the spell rules are very vague. It is a very powerful spell if it can effect anything metal,
Yeah, that seems an entirely appropriate use of heat metal. "A weapon" or "A suit of metal armor" are the examples given in the spell description, so "a shield" seems completely in line there.
One thing to note - the spell says the NPC drops the object IF IT CAN. It's quite likely that armor, for example, CAN'T be dropped easily, and the NPC would have to take the alternate effect (it has disadvantage on Attack rolls and Ability Checks until the start of your next turn.) A shield could go either way, you could either let them drop it or say that the shield is strapped to the NPC and it can't drop it easily. (Usually it takes an action to take off a shield.)
It's certainly a useful spell - but not, like, gamebreakingly powerful or anything. Usually it does 2d8 damage (not too much, not too little) and then usually either forces the NPC to switch to a different weapon OR take a penalty to AC (if it's dropping a shield or armor) OR open itself up for 2d8 bonus damage each turn if it immediately picks up the thing it dropped.
Alternately, if you don't like having the shield separated from the wielder for plot reasons, your players have now given you an excellent IN-CHARACTER reason for the powerful NPC to find a way not to be affected again! Maybe the NPC enchants the shield with some anti-magic or frost defenses, maybe they strap it in so that it can't be dropped, or something. After all, the NPC's been burned once now. Fool me once, shame on me, fool me twice - won't get fooled again.
An interesting case might be if the shield is actually dominating the NPC, who might well want to drop it but cannot. In that scenario, I would let the NPC make a Wisdom save against the domination every round they take damage.