The druid class in Dungeons & Dragons 5e has the unusual restriction of not being able to wear metal armor or use metal shields. This restricts the kind of gear that they can wear in combat, which often forces them to rely on their spells and class abilities for protection. The creators of D&D have addressed what would happen if a druid did put on a suit of metal armor and why that decision was made, as it has roots in the history of the game, with D&D's druids referencing real historical organizations, being based on Celtic priests.
Druids have always been among the stronger D&D classes throughout the editions. While they can't wear metal armor, they seldom need to, as druids have historically had the ability to transform into animals and even elementals. These alternate forms are often powerful in their own right and don't require armor to be a threat on the battlefield, as they provide extra hit points and natural attacks. Druids are also primary spellcasters, combining the healing and buffing spells of the cleric (with the notable exception of resurrection magic), with the summoning and elemental attack spells of the wizard. The fact that druids often had animal companions (or a means to charm animals to their side) meant that they were a force to be reckoned with and could often go toe-to-toe with the arcane casters, especially when in their natural environment.
Much like how D&D clerics once couldn't use swords, the druid class members share a strange restriction regarding metal armor. This is despite the fact that they can often use weapons with metal parts, like sickles and scimitars. The fact that druids consider metal armor taboo is also strange for evil members of the class, as evil druids have no problem incinerating settlements that harm nature, but would balk at the prospect of wearing a steel helmet into battle. Druids have a complicated history with metal items that continues into the current edition of D&D, but the rules aren't quite as restrictive as they used to be.
D&D's Druids Have A History With Metal Items
The druids are one of D&D's hardest classes for new players to get to grips with, due to all of the restrictions they have and the powers they get, which has been a problem in each edition. The idea of druids not being able to use certain items is not a new one. In their original incarnation in Eldritch Wizardry and the first Player's Handbook, druids could only wear leather armor and use wooden shields, but they could use any weapon they want. In AD&D, the druid weapon list was restricted to club, sickle, dart, spear, dagger, scimitar, sling, and staff, but they could now use padded and hide armor.
In D&D 3e, druids started out with proficiency in club, dagger, dart, quarterstaff, scimitar, sickle, shortspear, sling, and spear. It's possible for a druid to multiclass or gain proficiencies in another manner, There main restriction comes in the form of armor, as they cannot wear metal armor or use metal shields. If they do, then it interferes with their ability to cast spells. D&D's armored wizards and spellcasters have always been strong, which is why these restrictions used to exist. This restriction wasn't as much of an issue in 3e, as most druids took the Natural Spell feat as soon as possible, allowing them to cast spells while transformed using their Wild Shape feature, so it was often better to just transform into something bulky when in combat. Natural Spell was so popular and powerful that it was taken out of the reach of most players, as druids in D&D 5e don't get a similar ability until they hit level 18.
What Happens When A Druid Puts On Metal Armor In D&D 5e?
D&D 5e has the least restrictive rules of any edition of D&D. Gone are the days when D&D's paladins had to be lawful good and spellcasters had to jump through hoops to wear any kind of armor. This doesn't extend to druids, as one thing the 5e Player's Handbook is very clear on is druid's use of metal items. In 5e, druids start out with proficiency in Clubs, daggers, darts, javelins, maces, quarterstaffs, scimitars, sickles, slings, and spears, as well as light armor, medium armor, and shields. It's specified that druids will not wear metal armor or use shields made out of metal, though the penalty for doing so isn't explained.
The question of what exactly happens when druids put on metal armor in D&D 5e was addressed in a Sage Advice column, which has been archived on D&D Beyond. Unlike in previous editions of the game, the presence of metal armor or shields does not interfere with magic or a D&D druid's ability to cast spells. Instead, the decision to not wear metal equipment is due to personal beliefs, as druids share a much stronger connection to the natural world than other characters. As such, they prefer to use gear made from natural substances, such as leather, fur, or wood. D&D Lead Rules Designer Jeremy Crawford also clarified on Twitter that multiclassing doesn't bypass these rules, so it's not case of proficiency affecting the druid's ability to wear armor.
A druid can theoretically wear metal armor and have it not affect their class abilities, nor will it make them overpowered if they gained the ability to don a suit of plate mail. Druid is a primary spellcaster class with a focus on shapeshifting, healing, and powerful elemental combat spells. This means that most druids will either be fighting from the back row with spells, or turning into a wolf and leaping to the front lines, where they would not gain the bonuses of their armor anyway. A DM who wants a D&D homebrew setting that changes the rules, allowing druids to wear heavy armor without in-universe ramifications, then they can do so without breaking the game. The restricted druid weapon and armor list is a holdover from a different era, but it also reflects the core tenant of the class, as they eschew unnatural elements as much as possible. The Dungeons & Dragons 5e druids don't wear armor because they choose not to wear it, not because they are forced to.
Source: D&D Beyond, Jeremy Crawford/Twitter