Does the throttle stop feeding the fuel required to idle the engine, just because you’re in gear and coasting?
Your mechanical losses are greater in gear as you’re turning the engine more quickly. Do you burn less fuel? Which is the greater effect?
When on overrun (in gear, but no pedal), a modern engine will inject zero fuel and use the flywheel effect of the engine to keep it turning. Only once the engine approaches idle speed will the idle governor kick in and start putting fuel in. You would use much less fuel by ‘coasting’ in gear, accelerating gently when starting to slow down a bit and then coasting in gear again than you would be cutting the engine and starting it if you need power steering! Not to mention that losing power steering, brake servo etc is not the most sensible idea….