This question stems from another.
Does a shield count as "armor" with regard to the proficiency requirements listed under Armor Proficieny?
Yes, a shield retains the same Armor Proficiency requirements that Armor has.
PHB 144 states:
Anyone can put on a suit of armor or strap a shield to an arm...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.
This description under Armor Proficiency includes shields in the first sentence and therefore the description of non-proficient use includes both Shields and Armor.
Note: There is still a difference in shields vs worn armor when looking at things like Mage Armor, Unarmored Defense, etc. This is a discussion on proficiency, not on how it interacts with other capabilities.