Everything that can be airgapped probably should be. Healthcare, education, public safety, social services, public administration. Anything that touches massive troves of sensitive data for an entire country or continent.
Non trivial amounts of industrial and civil infrastructure, sometimes the air gapping is also because there's no viable connectivity more so than security reasons (or you have to assume flaky network so the system is designed to operate as separated island 99% of time - for unlikely example Chick-Fil-A has such requirements for their k8s clusters running individual restaurants.
Sometimes it's also simpler to physically air gap a system than to deal with more complex security.
