For me, the PHB is really clear:

(druids will not wear armor or use shields made of metal)

I would essentially equate this with druids not being proficient with metallic armor, and apply this text, also from the PHB, emphasis mine:

Armor Proficiency. Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor's use know how to wear it effectively, however. Your class gives you proficiency with certain types of armor. If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can't cast spells.