Laurefindel
Legend
Sometimes I start to wonder if the intent of this "no metal" limitation was lost over the many editions of D&D. I think in 2nd edition they just assumed it was a creative way of restricting them from using specific types of armor like full plate that were by default made of metal, but nowadays D&D has introduced so many special materials you could feasibly create a set of full plate armor that is made of chitin and provide similar protection.
I for one see the druid's "no metal armor" restriction as a vestige of OD&D and AD&D. It was always presented as a taboo of a specific Celtic-like human religion (when other races apart from half elves couldn't play a Druid), alongside the promotion of true neutrality and a rigid hierarchy of druids and archdruids (IIRC, a Druid could not progress past 12th level - or get to 12th level? - without taking somebody else's place).
druids were made as a sub-class of cleric, and in 2e AD&D an example of a priest of a specific mythos. Since cleric could wear heavy (metal) armor, the "no metal armor" taboo came to balance this "sub-cleric" class alongside other abilities.
with the Druid now decoupled from humans of a pseudo-Celtic religion (or any religion for that matter), the "no metal armor" taboo becomes harder to justify.