Homebrew perhaps but definitely not the official modules.
Maybe not in the ones you played in, but again in my experience you can.
HotDG: At level 3 the party can gain ~2500 total gp from a dungeon, plus 250gp per player as a reward for finding [spoiler location] at level 2 and then 150gp per player for investigating said [location] at level 3. With a party of 4 this sums to 4100gp, easily enough to buy 2 Plate Armors is loot is shared. Unless your DM declares that "Plate Armor isn't available for purchasing" :\
Similar for LMoP and W:DH - you can get enough to buy Plate around levels 4-6. So yes definitely in at least some official modules you can get Plate Armor.
Not sure what tier system you're using but +1 light and medium armors are still tier 1 magic items. (I'm using the game's own tier definitions, T1 being level 1-4 of play).
These are tiers I made up, corresponding to the armor progression. Level 1 characters start with Leather / Chain Shirt / Chain Mail = "Tier 1," with each next step of armor increasing a tier. I admit this is confusing, given the official Adventuring Tiers of Play.
Edit: The fact that Larian includes Ring Mail in the game instead of Chain Mail really shows they don't understand D&D armor. I've literally never seen a player wear ring mail.
Edit to your edit:
As for "nerfing STR fighters" you're electing to ignore shields. Yes, you can go 2-handed for more damage and sacrifice AC, but your average sword and board fighter tends to have a better AC than an 18 DEX rogue in +1 studded leather. Even with what is currently in game, ring mail + shield is a 17 AC putting them on par with everyone else and still well-suited to the tank role with second wind.
Sure, I am ignoring shields. But to be fair, classes that make use of medium or light armor (clerics, fighters, and rangers in particular) can also use shields. So they can also get that +2 AC buff. A Dex fighter might want studded leather, a shield, and a rapier. And regardless of what other classes can get, a str-fighter without Plate Armor is less tanky than a fighter with it.
