From an actual operations practice, approach MODE is usually activated when ATC clears the IFR flight for the approach. That’s when the controller would expect the aircraft to be slowing to final approach speed and configuration for landing.
Prior to this, ATC - would be providing vectors, speed, and or you’d be following the STAR so you’d most likely never get close to the magenta “D” - decelerate point where the aircraft would begin slowing, as the configuration allows, to vAPP.