Undrave
Legend
The Druid is nonproficient with metal armor. A breastplate made out of a hypothetical adamant crystal (like made out of sapphire or ruby) is proficient. But a metal breastplate, no.
The penalty for nonproficiency would be no spellcasting.
You're the only one reading it that way.In the case of the Druid class, the armor proficiency is explicitly material based. No metal.
Otherwise, the proficiency would have said, "light armor and hide armor". But other armors are proficient, as long as they are not metal.