If your network isn't air gapped, then it's vulnerable, and attackers will side step the "read only" path to affect control system changes.
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
You can't drive my car simply by pointing a webcam on the steering wheel.
My position is that if there's any connection, of any practical type, between the control system and the outside world, an attacker will be able to find ways to break things.
Even if it just boils down to "an attacker made our dashboards report numbers wrong and so we closed a valve to prevent an issue that didn't exist, and inadvertently destroyed our own product", the outcome is the same.
Temperature sensor is a sensor. Refrigeration compressor motor is an actuator.
GPS positioning of a car is a sensor. Motor to turn the wheel is an actuator.
Aircraft altimeter is a sensor. Motors to control control surfaces are actuators.
And so on.
Some devices are sensors, like temperature sensors. It's not that they're "read only", it's that... You can't override temperature. For that, you need other devices. Like a switch to a motor that operates refrigeration compressor. That would be an actuator. If you put the data from the sensor online, the worst case scenario is someone unauthorized gets your data. If you build interface to control an actuator online... You can read the article up top.
The problem is that all major cloud providers (AWS and Azure for sure, and I'm sure Google as well) have a large number of services targeting IoT devices, both to read and control. So default architectural decision is "We'll just plug these into AWS IoT, it's secure, reliable, highly available, and has all the features we need, all we have to do is to configure what we want and we're all set". Which in reality is not that secure just because of how IoT is designed. But AWS/Azure obviously push for these solutions because it gives them more revenue.