side channel exfiltration techniques like "fansmitter"[1] and "diskfiltration"[2] or the well known TEMPEST attacks are more interesting challenges in air gapped designs than relying on a very basic security boundary violation. (and solved in very much the same way, by having the secure terminals be as dumb as bricks as possible, with extremely limited known inputs and outputs, preventing the insertion of unclassified electronics in the secure zone through policy, and enforcing it through physical searches, RF hardening the zone with a faraday cage, absence of windows, etc... the "Technical Specifications for Construction and Management of Sensitive Compartmented Information Facilities"[3] is a great ressource for those interested in such designs)
incidentally, there are awfully little true air gapped networks (which are not ICSes and/or assorted single purpose hosts/networks whose absence of network connectivity is not a primary design feature) in the wild. they're operationally heavy precisely because of their nature, and outside of very specific situations, their security is not perceived as cost efficient by management, for good reasons.
[1]https://arxiv.org/abs/1606.05915 [2]https://arxiv.org/abs/1608.03431 [3]https://fas.org/irp/dni/icd/ics-705-ts.pdf