Asking for a friend... what is the original reason for druids' metal aversion?

[Arkhios]

Time to retire the name and rebuild the wildshaping class into something that isn't a primary caster, so the wildshapes can actually be fun and useful (a bit like the druid from Honor Among Thieves). Then, they can stick with the whole 'non-metal' armor thing, because 'metal can't be subsumed into the new form, for reasons.'

My wife is about to start her own game at her workplace as a weekly recreational activity for chosen clients (mostly severely disabled people etc.) and she made that exact decision: druid in her game won't be a spellcaster, but rather a wildshaping class that borrows from other classes' abilities depending on the chosen form. e.g. a bear form deals extra damage akin to the barbarian's rage or dueling fighting style.