What else would you exfiltrate/steal besides passphrases and private keys? Serial numbers/IPs/hostnames of any discovered devices on the air-gapped network?

Send to the airgapped computer(s) software updates and new commands to run? 20 bytes per second is enough for that, or is it bits? A shellcode is about 40bytes or less.

I guess if you want to guard against this the reasonable thing to do would be to physically take out the mic and speakers of any to-be-secure-computers. Or have one computer listen in on these high frequencies on the perimeter or whatever. Would be interesting to discover chats.