As a specific example, From playing a Druid in DCI Adventurers League games (specifically "The Quest for Sporedome") I have a magic Item cert for a set of Half-Plate of Poison Resistance (Highest AC medium armor).  The armor is explicitly usable by Druids, as it specifies on the cert that the plate is made from petrified giant mushrooms.

And seeing as these adventure modules are now being sold, officially, via WotC's Dungeon Master DriveThruRPG site, they are definitively published official first party supplements.

Further Information: It's worth noting, I play a Circle of the Moon Druid, and so the only time the extra 3 ac from having half-plate vs hide apply is when I'm either standing back and casting just for a change of pace, or during the surprise round of an ambush.