An APT hacking group known as GoldenJackal has successfully breached air-gapped government systems in Europe using two custom toolsets to steal sensitive data, like emails, encryption keys, images, archives, and documents.
Technically, it's a USB drive that bridged the gap. It doesn't somehow nullify the air gap, it just stops working when you break that gap. So the air gap is still useful if you can stop idiots with infected flash drives breaching it.
It's not air gapped if you allow flash drives to be plugged in. I'm not sure why anyone would refer to a system that allows flash drives as "air gapped". If they were prohibited by policy, but still able to be connected to open USB ports and recognized by the system, that's not good enough. The USB ports need to be disabled.