Most control systems are not airgapped, because most people don't think of themselves as targets of that level of attack. Stuxnet was targeting national security infrastructure, which is much more likely to be airgapped, but your local powerplant doesn't (at present) think of themselves as national security infrastructure, so they don't take the same level of precautions.

Note: this only describes cases I'm familiar with, in the US. I bet that some countries with more experience on the receiving end of cyber-warfare (e.g. Ukraine) are better.

The airgap question comes up every time an industrial system is attacked. The thing is that these days all systems are connected via several layers to the outside world. Because a modern oil refinery is of no use if you cannot track what product is being made at that moment. And the people tracking the data are often in completely different locations or even different countries.

The main line of defense is these days is "layers of an onion" network with (physical) controls such that data easily can get out to a higher layer but that it is very difficult to get in from a higher layer back down into a deeper layer.

A completely airgapped network is not practical anymore because the alternative is even worse: nobody wants to have dozens or 100ths of operators, maintenance engineers and 3rd party contractors running around the facility with usb sticks because there is no network to move stuff around. If you have a network then you can control where the data comes into your network, who copies the data to where and what data is visible for which user.

They'll be somewhat networked at least together and if the operators want any convenience they'll be on a network with most other systems just so they can be monitored more easily.