O Heat Metal a descrição do feitiço diz:
Choose a manufactured metal object, [...] Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
[...] If it doesn’t drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.
Se um personagem montado usando armadura de metal tiver metal quente lançada sobre ele, sua montaria seria considerada como estando em contato com a Armadura de placas (ou qualquer outra armadura, como Scale Mail)?
Scale Mail. This armor consists of a coat and leggings (and perhaps a separate skirt) of leather covered with overlapping pieces of metal, much like the scales of a fish. The suit includes gauntlets.
[...]
Plate. Plate consists of shaped, interlocking metal plates to cover the entire body. A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor. Buckles and straps distribute the weight over the body.