My 60's era cars all had water flow valves that turned coolant flow off to the heater core. But all my newer stuff has continuous flow through the heater core (its often used as the thermostat bypass, in fact) and then use air blend doors to regulate cabin heat.
Blend doors produce an almost-instant change in temp wwhen the driver moves the heat selector, the old water-valve systems take a while for the hot water to get through the core, heat the metal, and start producing warm air so the driver tends to over shoot, get too hot, turn it down too far and it takes a while to cool off, then it gets too cold... lather/rinse/repeat. When I daily-drove the older cars, I always had a certain spot on the temp slider that I aimed for and then made very small adjustments.