Hi Grant - welcome to the community! What specific device handler / driver are you referring to?
SmartThings does not have a supportedFanSpeeds attribute - it was just something we proposed to them.
Hubitat eventually added a supportedFanSpeeds attribute to Hub version 2.2.6 (March 2021), but as of my review in May there weren’t any built-in devices that reported this attribute.
Edit:
The Thermostat tiles respect the supportedThermostatModes and supportedThermostatFanModes attributes already. ![]()