For many years I've just viewed all of my devices as possibly compromised. It's one of the reasons I've been very down on cryptocurrencies in general. I don't actually see USB as something that can maintain a true, robust airgap, because the amount of data transferred is not inspectable.
In my view, the best use of an airgapped machine would be for storage of extremely dense and sensitive information such as cryptographic keys. Signing or encryption should be accomplished through an inspectable data channel requiring manual interaction such as QR codes. When every bit in and out of a machine serves a purpose, it's much less likely to leak.
Example: show a qr code to a camera on the airgapped machine and get a qr code on the screen containing the signature of the data presented to it. There is very little room for nefarious code execution or data transmission.
In my view, the best use of an airgapped machine would be for storage of extremely dense and sensitive information such as cryptographic keys. Signing or encryption should be accomplished through an inspectable data channel requiring manual interaction such as QR codes. When every bit in and out of a machine serves a purpose, it's much less likely to leak.
Example: show a qr code to a camera on the airgapped machine and get a qr code on the screen containing the signature of the data presented to it. There is very little room for nefarious code execution or data transmission.