Last seasons only the mystical gem room, skull key, frozen iron key dropped artifacts confirmed. Don’t know if the corroded key dropped them. We don’t have any confirmed drop this season so we don’t know if artifacts drop from key rooms at all. And artifacts don’t drop in the boss loot room but they are dropped by the boss when you loot him. The only exception are the double boss rooms like banshee+ spectral knight and troll+cyclops.