The Fuel cut off that may happen from coasting in gear would occur so infrequently
in my car that that technique would waste gobs of fuel every time i drove. I live in a mountainous region and yet my car never will coast downhill in 5th gear above 2000 rpm, Most of the time i drive at 1000 rpm meaning the fuel cut off will rarely happen in a unlimited amount of miles of driving, for me in my car.
In gear coasting sucks the distance out of the coast, My estimate is 10-15% less coast distance with the car in gear, meaning the throttle needs to be engaged that much earlier.
I suspect the technique works best in automatic transmission cars, what choice do you have with a auto,in most cases.
I have been unable to detect the RPM at which the fuel cut off occurs on my car.