I suppose I shouldn't complain about it after complaining so much about alignment restrictions in 3.X/PF1, but I don't see the metal armor restriction as nearly as baseless as weapons restrictions for Clerics or... well, practically any/every alignment restriction for classes other than Cleric/Paladin.
It's a little weird they're allowed metal tools and weapons, but denied metal armor, but it's not totally weird; you wield a weapon in your hand only when you need it, but you wear armor whenever you might need it. Weapons make you more dangerous predator/prey, but metal armor-- realistically-- makes you practically invulnerable to most animals' attacks.
I'm not a purist about this. I'd allow a Druid PC to waive this restriction with a good reason and a small cost. In 5e, being proficient in Medium armor means being qualified for the Heavy Armor Proficiency feat. Take the feat, with a narrative justification... sure, wear whatever not unnatural armor you want.
In a game with (fixed progression) multiclassing and some kind of feat/slot option, I'd allow any Druid/armor-wearing class to buy metal armor with a feat or two.