It is just too confusing and counter-intuitive to have Light Plate (Tier 2 Armor) require Plate Proficiency (Tier 3 Proficiency) and Masterwork Chain (Tier 3 Armor) require Chain Proficiency (Tier 2 Proficiency).  I suggest:

Light Armor (Starting Armor and Leather): No Proficiency Required.
Medium Armor (Chain and Light Plate): Medium Armor Proficiency Required.
Heavy Armor (Masterwork Chain and Plate): Heavy Armor Proficiency Required.

Note: Rusty Armor could either be Light or Medium Armor, depending on where the devs wanted it to fit.

This would also solve the problem of unit designs for sides that currently have Light Plate and/or Masterwork Chain technologies.

I don't mind Shields being exempt from Armor Proficiency, as it gives those without any Proficiency the opportunity to increase their defense.  But if the devs wanted to I wouldn't mind the different shields being added to the appropriate Light/Medium/Heavy Armor tier classification.  This would mean Mages could still use Light Shields but not the heavier shields, and the classes with Medium Armor Proficiency only could use Light or Medium Shields but not Heavy Shields.

Note2: Light Plate could still use Plate level for initiative penalties, Masterwork Chain could still use Chain level for initiative penalties, and Rusty Armor could still use Plate level for initiative penalties, to provide some extra differentiation and to have a reason to choose Masterwork Chain and to make Light Plate not a no-brainer.  Or you could just equalise initiative penalties to Light (no penalty), Medium (Medium Penalty), Heavy (Heavy Penalty) but make Light Plate require more Metal than Chain but less than Masterwork Chain, and Masterwork Chain require more Metal than Light Plate but less than Plate (and the same hierarchy of costs for gold cost of champion armor).  But Tiers (No Proficiency for Tier 1 Armor, Medium Armor Proficiency for Tier 2 Armor, and Heavy Armor Proficiency for Tier 3 Armor) seems like it's the least confusing and most intuitive way to go for Armor Proficiency.

Devs, could you implement this?