Disabling the spindle override is determined by the machine builder, not Fanuc on newer controls.
Adjusting the spindle speed during a threading cycle on older controls usually results in a slightly imperfect thread form due to synchronization errors between the Z servo and the spindle drive. The Z servo velocity is only updated once per spindle revolution. The amount of error depends on how quickly and how far the override is turned and the spindle's response speed to override input. Generally it is advised to not adjust the override during threading operations.