D&D 5E Question about shield proficiency

[jaelis]

From what I can see on the armor section, shields are a form of armor.

They are mentioned under Armor Proficiency on page 144.

"Anyone can put on a suit of armor or strap a shield to an arm. Only those proficient in the armor's use know how to wear it effectively."

Additionally, shield is listed as a type of armor on the chart on page 145, which I think seals the deal.

So without shield proficiency you suffer the listed penalties as it is armor.

I agree with the conclusion here, but whether a shield "counts" as armor is actually a bit subtle. For instance, in regards to mage armor the Sage Advice says "Shields are grouped with armor in the equipment rules in the Player’s Handbook, but various game features distinguish between the armor you wear and a shield you wield." And goes on to explain that shields do work with mage armor.

So you can't just make a blanket assertion that shields count as armor.

But the armor proficiency discussion explicitly includes shields, so I think the reasonable inference is that that shields are meant to be included there. Mike Mearls confirms on twitter:

Q: PHB:144 there are drawbacks if you wear armor you are not proficient with, what about a shield
A: shield has same drawbacks as armor