If I were a D&D character, I don't think I would qualify for shield proficiency. Nevertheless, I understand the basic principle of holding the shield between me and the enemy. So I wouldn't get the shield bonus to AC (lack of proficiency), but getting some benefit from it because the shield is magical? That doesn't break my suspension of disbelieve.
Oddly enough, if you used
two hands and held a shield between you and an attacker, basically taking the Use an Object action, as a DM I would allow you a half-cover bonus to your AC and under
that situation allow the +X of a magical shield to apply (maybe... I'm not absolutely certain, but at least you could argue it in this scenario).
But, this requires you to "use" the shield since you lack proficiency, and sacrificing your action to gain a bonus to your AC is reasonable (you are probably better off just Dodging, but whatever
shrug).
If, you are trying to hold the shield in one hand without having it donned, you aren't using it to protect you in any fashion, and so you should
not get any bonus, even from the magic. Hell, you might as well be holding a Breastplate +1 in your hand and argue you should get the +1 bonus from the breastplate! Sorry, it just doesn't make sense to me and does break my suspension of disbelief. (To be clear, if you held the breastplate and took the Use an Object action to put it between yourself and an attacker--maybe,
maybe I would allow the +1 for the magic armor... maybe.)
Now the rules a a bit weird about differentiating "holding" and "wielding", one being a free interaction and the other a full action. When someone is holding a shield in combat, I'd assume they're holding it by the handle rather than wearing it like a hat, so really the action of donning a shield is about getting ready to properly block a blow. So an ordinary (unready) shield gives no protection even to a proficient user, but in the case of a magical one, its magic would make it partially useful? Again, it doesn't break my suspension of disbelief that hard.
Yeah, no I still wouldn't follow this. If you had a shield +X slung on your back, you aren't going to get the +X to your AC. AC is too important in 5E due to BA, etc. so I wouldn't allow it.
Like every other magic item, if you want the benefit the item confers, you must use the item as it was intended to be used.
And what, handing out a
legendary magic item might be broken in some situation??? SHOCKER!

[edit] ok, magic shields aren't that rare. You can

me back!
Nah, that was about the Robe of the Archmagi. The Shield +2 in the OP is a rare item. We are in tier 4, but have two PCs with them.
shrug