you'd be surprised.
when specifications get passed around in the endless maze of defense affiliated sub contractors and sub sub contractors, they may be implemented in all kind of creative ways.
and i entirely agree that the operational issues of enforcing the air gap are by far the most challenging cost in this kind of environment.
the culture of business environments is by and large ill suited to the kind of strict enforcement these environment require.
if RF leaks are part of your threat model, then i would definitely not class them as "proximity attacks". without adequate shielding, antenna black magic and a rented apartment on the other side of the street work perfectly well to capture signals of interest.