I have heard it said that 5e does not balance its encounters with respect to magic items; in other words it does not consider possession of magic items when defining appropriately challenging encounters. But what about mundane items? Specifically, plate mail. Due to its cost, it stands out dramatically from other, mundane items.
I have read numerous forum posts discussing the "correct" rarity of such an item, and what character levels are fitting for PCs acquiring it. What troubles me about this is plate is a significant part of the Heavy Armor proficiency - if plate (and to a lesser extent, splint) is not available (either by gold restrictions or the DM saying it's hard to find) then Heavy Armor proficiency becomes much less valuable to a PC. Light Armor users easily get to 16 AC, and Medium Armor users can get to 17 at half of the cost of plate. Now, plate is 18 AC, but it comes at a great financial cost, Strength restrictions, and Stealth disadvantage. Seemingly, the only real benefit is that they can also get to 17 AC cheaper/more easily than any other armor proficiency.
Still, in many campaigns plate is out of reach for the first five (or more) levels, making the Heavy Armor proficiency relatively weak for any classes that offer it. Even once that point is reached, AC tends to be less important than raw HP or magic resistance/saves at later levels - so by the time plate becomes available, it matters less than it would have earlier.
In short, I identify that the availability of plate mail as a huge "swing factor" in the value of Heavy Armor proficiency.
Do any 5e official sources (source books, designer statements, or Adventurer's League) offer any indication on the appropriate time to allow PCs to have plate?