But they do have medium armor proficiency, don't they? That suggests to me that designers expect them to wear such armor.Them not being able to wear metal armour is an actual rule, and it effectively prevents them from wearing medium armour. So if this is intended to be balanced, just allowing them to ignore this restriction would be basically the same than giving any other class a free tier of amour proficiency. My current game has altered armour rules anyway, but I scaled the druids with the assumption that they're effectively meant to be light armour wearers.
But all medium armours except hide which is rubbish are made of metal. And the restriction is part of the rule so probably it is meant to mean something?But they do have medium armor proficiency, don't they? That suggests to me that designers expect them to wear such armor.
The "no metal" requirement sounds to my ears more like fluff than mechanics.
Scale Mail is not always metal, and a breastplate can be made of Chitin.But all medium armours except hide which is rubbish are made of metal. And the restriction is part of the rule so probably it is meant to mean something?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.