Plaguescarred
Hero
IMO a shield would not stack as its an armor. If it was not an armor, then there would be no penalty for wearring one while not proficient;
BR44 Armor Proficiency: If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.
BR44 Armor Proficiency: If you wear armor that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.