Hmm. While I think I get the idea of this feat, it strikes me as being rather overpowered.

A shield is a crucial piece of equipment to many martial combatants. They pay good money for its effect, and sacrifice a second hand to use it. To completely destroy one (regardless of material composition, apparently) just because the attacker rolled a 20 to attack seems rather unreasonable.

And considering that all it takes to do it is a melee attack; that means the shield can be destroyed with a simple club; a dagger; barehanded; even with a cantrip like Shocking Grasp! A level 5 monk with no weapon, provided he's strong enough, could be surrounded by 4 knights wearing shields, yet could potentially destroy ALL of them in a single turn with a simple Flurry of Blows. Again... that seems unreasonable.

What if, instead, the feat allowed knocking the shield off the target's arm? While they might be able to pick it up for free, they'd need to spend an action to don it again during battle. And at least they don't have their elven-crafted solid steel heirloom shield utterly destroyed through a single attack.

Historically speaking, Shields used to break all the time. In fact, that was a not uncommon tactic to intentionally break the enemy’s shield so they would be left with a handful of splinter Med wood and be easy pickings. Some duels would even be fought “to X Shields” (3 was a common number). Instead of “1st blood” or “to the death,” they would fight until one combatant had run out of unbroken Shields.

Most Shields we’re never “heirlooms,” nor were many of them made of solid steel. The vast majority were wooden with a steel band and buckler (center cap) and a single center-grip. They were expected to get trashed because, better it than me. Even the heirlooms show signs of repeated repairs over generations.

Besides, this feat was written for PCs to use, and they mostly fight monsters with a total life expectancy of proximately 30 seconds….