It's true, pretty much all modern injected cars (petrol or diesel) have an over-run fuel cut-off feature and yes, it's controlled by the ECU. By "modern", I'm talking about stuff that has generally been manufactured in the last 20 years.
On over-run, all petrol engines (and possibly some diesels these days?) generate a ferocious vacuum in the inlet manifold and cylinders. They'll do that with or without the over-run fuel cut-off. It's just because there's a throttle plate blocking the air off. It does them no harm.
You are GENERALLY wasting your time coasting in a modern car, BUT...
here's the problem.
If you're doing (say) 50 along a road with a slight downward slope and you lift off the throttle in the highest gear you've got, your car will use zero fuel, but the drag of the gearbox driving the engine (engine braking) will slow it down somewhat. If you're doing the same speed on the same road and you kick it into neutral, your engine will burn whatever fuel it would burn when idling (instead of zero fuel), BUT, there will be no engine braking, so you'll go further.
As for which is going to work out the most economical, that's a tough one! It will vary with every set of conditions. If the traffic is slowing down at least as fast as the car would have done under engine braking anyway, then in-gear is definitely the way to go as you will use no fuel. If you're going down a hill and want to maintain a constant speed, then coasting MIGHT be the way to go. As Peter N says, if you have a trip computer that gives istantaneous consumption readings, you have the luxury of trying it both ways and seeing. My personal experience is that it is generally best to decelerate in the highest gear you can (unless you need the engine braking). It's pretty rare that coasting is the better bet.
Obviously, as part of the responsible motoring "never-run-with-scissors" obligation, I could never advise anyone to coast in neutral!