It's being called a data diode (diodes usually allows current in one direction only).
It can be implemented with a fiber optics cable with only the transmitter on the secure side, a laser+photo cell for real air gap, cutting the rx cable in a rs232 or having a microcontroller with an output pin connected to another controllers input pin. By handling it with hardware instead of software you get a very secure partition between the trusted and untrusted side.
It's used to monitor for example industrial systems and power plants.
Wikipedia calls it unidirectional network: https://en.wikipedia.org/wiki/Unidirectional_network
Yes, this is exactly the kind of approach I'm referring to when I say it's possible (though not trivial to implement), but it still doesn't prevent attacks. Air France 447 crashed with extraordinary loss of life because the pilots responded incorrectly to reported sensor readings. Any scenario where authorised staff are able to affect changes in a control system is vulnerable. The attack vectors just change to suit the target system.
AF447 wasn't a security breach. You're talking about an entire separate class of human-machine interaction problem that has little relevance to the lack of security around access to OT operational surfaces.
All fancier than a camera pointed at the thermostat readout I’m sure but is the laser & photocell anywhere close to similar in concept?