While I don't necessarily agree that plate is necessary super early game-balance wise (I do like the aesthetic of plate more than chain for say, Paladins, so wouldn't be opposed to getting standard plate early for a 2h Paladin build once Paladin becomes available).

But re: availability in official modules, I started a Ghosts of Saltmarsh campaign (that didn't get past lvl 3 because scheduling conflicts), but we found plate mail in the basement of the haunted house, at level 2. Our DM was flabbergasted that there was a suit of plate in the basement that early. He hadn't looked to closely at that particular room while prepping for the session, so it came as as much a surprise for him as it did for us. My Maul-wielding Paladin was VERY pleased. So it can be available early in officially published stuff too.