FrogReaver
The most respectful and polite poster ever
Actually it is, but indirectly.
A shield, like any piece of armor, must be donned to benefit from it, and must be doffed to remove the benefit. (See the Donning and Doffing Armor table in the Player's Basic Rules.)
Dropping an object you are holding is not specifically defined in the rules, but is usually ruled as not an action, as there is nothing in the rules for Interact with an Object or Use an Object that require you to use it to drop an item you are holding. If you could remove the benefit of a shield this way, or by using the free 'interact with an object' action available to a character each turn to put the shield away, just as a character can use that action to sheathe or draw a weapon (see the list of Interacting With Objects Around You in the Player's Basic Rules), then you could argue that the rules treat 'holding' and 'wielding' a shield as the same.
Since dropping an item you are holding does not require an action, and removing a shield you are wielding does require an action and does not qualify for the Interact with an Object action, wielding a shield is not the same as holding it in the rules as written. A DM could certainly rule that a character could use Interact with an Object to 'put away' a shield in contradiction of the Donning and Doffing Armor table, but that doesn't change the rules as written.
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Pauper
Good. Followup position. the Armor Proficiency rule only calls out "wearing" armor while not proficient giving you all the bad effects. Does one actually wear a shield?