Industrial equipment control computers mostly - the computer is part of the machine and cannot be upgraded. You see this quite a lot outside China too.
Using Windows XP on those devices often gets very bad rep but is there a viable alternative for machines that are air gapped and technically won't be upgradable for several decades?
source: https://www.cyberscoop.com/duo-labs-air-gap-radio-mikhail-da...
Links to: https://duo.com/labs/research/finding-radio-sidechannels
I'm aware of the "over the air" methods to _read_ data from air-gapped systems (The blinking hard-drive light!), however I was referring to doing damage to industrial systems running legacy software, where you usually find this (think a big machine with a "terminal" attached to it that just controls the hardware and has no inputs).