Yes, this is illegal! Don’t Do It.

 BC Motor Vehicle Act.

Coasting down grade

197  When travelling down grade a driver must not coast with the gears of the vehicle in neutral or the clutch disengaged.

Why do you think this is so? Maybe because in neutral, you have the inability to speed up to potentially avoid a collision or situation. Your car will only glide with gravity and will roll down the hill according to inertia, physics, and all of that stuff; rather than being in proper control by a driver.