I dunno, if I got shot within reasonable range with 12ga birdshot, I think I'd be out of the fight. If it goes through plywood, I'm pretty sure a T shirt isnt much of a barrier...
Penetration of plywood is not equal to sufficient penetration to stop an attacker.
Now, so called psychological stops, as in, "oh my god I just got shot, I am going to stop doing what got me shot", that could be accomplished with just about anything, maybe even a BB gun.
But we don't want to rely on that, because somebody who is determined, or has adrenaline masking their even knowing they got shot, or is under the influence of this or that or the other, might just keep coming at you until they are physiologically stopped. And that takes more penetration than stinking plywood.