Is a thermostat for underfloor heating necessary/mandatory?

Whether ERR is mandatory, I don't know. But modern heat pump systems (if no photovoltaics or solar thermal systems are present) should go directly into the underfloor heating, so they operate without a buffer tank (the screed is the buffer). The valves (if present) should all be fully open. The actuators, if present and connected, also require power in standby mode, by the way. Additionally, a hydraulic balancing and you have an efficient heating system – if properly planned. The ERR only disrupts the self-regulating effect of the underfloor heating, and corrections would still be possible via the heating circuit distributor. I can now only speak for heat pumps -> underfloor heating. ERR is a seasonal performance factor killer here.