Left shift key is stuck despite keyboard replacement

I think it would be something shorted on the logic board and not an os bug.

I don't believe it's an OS issue either. I also don't think it's a logic board issue; the logic board doesn't "control" the keyboard. I think there's an embedded controller in there which handles keyboard functionality and it just feeds the data back to the logic board. If the internal keyboard is disconnected and the USB keyboard works fine, that would prove that the internal keyboard is the issue.

I've replaced the keyboard TWICE at this point - problem is recurring. I suspect you're right about it not being OS - though I was hoping against hope. At this point is it likely a motherboard issue? There have been no other signs/issues but I'm running out of possible explanations.

Left out the crucial info: no, left shift is not stuck on external keyboard.

The electrical design of the keyboard is basically three sheets of plastic in a sandwich. The top and bottom sheets have traces on them while the middle sheet separates them by a tiny bit, with holes punched through where the contact points for the keys are. When you press a key, it presses down on the top sheet which pushes through the hole and contacts the bottom sheet, thus completing the circuit and registering a key press. On yours, either those contacts are stuck together or there's something in/under the left SHIFT key that's pressing down enough to register the key press. But since you replaced the keyboard twice the chances of this ACTUALLY being the problem are slim -- unless you just had bad luck with replacement keyboards. Where did you source your replacement keyboards from? Are they new Apple parts, or used from Joe Ebay or something?