As far as I'm aware, centrifugal feathering latches engage at the 900 rpm mark (±100 rpm) in most light twins. Ref. Robson from the Aviation Theory Centre series. I wonder at what CAS the propeller stops windmilling on a typical light twin, barring engine seizure? Could be close to VMC...
Also, a stationary propeller will give less drag than a windmilling propeller for piston powerd aircraft, as the energy to turn the engine is not being removed from the airflow. But a feathered propeller gives oh so much less drag again, which is why it is emphasised strongly in the EF drills.
O8