I was talking about a switch for the HVAC fan, not the nest itself. With the nest you have to fiddle through a bunch of screens to turn the fan on and off. It is a personal pain point of mine.
Nobody has ever offered this in a thermostat with the possible exception of the RadioThermostat CT30/3M50, which has a JSON API and lets you manually control the relays.
As I understand it any temperature control designed for big refrigerators or freezers has this features. Some even allow you to pre-configure fan cycles (i.e. on for x minutes, off for y, repeat) and turn the cycle on or off depending on temperature or compressor state.