Hriston
Dungeon Master of Middle-earth (He/him)
The consequence, as someone already pointed out up-thread, is that the druid is not proficient with metal armour, so all of the normal consequences apply. The game is not telling your character what to do. You are free to wear armour you are not proficient with. You will not, however, then be able to operate normally as a druid, casting spells, etc., and I think the DM would be justified in imposing harsher penalties depending on the exact nature of druidic orders in his/her world.