The designers deliberately chose not to lay out strict restrictions and guidelines for the adjudication of Stealth, and you're expecting a treatise on what constitutes metal armor and its ill effects for one class?Where I have a problem is a restriction which is presented as if it were a rule, but is not written clearly enough for an actual rule (which armors count as metal? what replacement options exist, if any? what happens if you do wear metal armor?), and when asked for clarification, the designers say "This is just a story thing, you can wear metal armor and it won't affect game balance if your DM is okay with it." Say what? Of course it affects game balance, metal armor is a lot better than non-metal armor! If druids were meant to be medium armor wearers, that ought to be spelled out in the rules and the non-metal restriction clearly presented as a non-binding story element. If not, that should also be spelled out, including details of precisely which armors are forbidden, and consequences imposed for wearing them.
Relax and consider the silver lining from the DM perspective. If a player's Druid is ignoring the lore and frequently leaning on the protection provided from metal armor, it means they're not Wildshaped and you have plenty of opportunities to do some real, meaningful damage.