A thermostat is not an on/off-controller but a proportional controller. The lower the temperature, the higher the flow rate. At least for thermostatic radiator valves this is true.
FWIW everywhere I've lived has an on/off thermostat, maybe it's regional
At least in Europe I've seen plenty of thermostats/boilers that support OpenTherm which allows the thermostat to set the boiler temperature instead of only switching on/off