Others have reported remote fuel level sensing technology at rental car returns. I don't know if it's an aftermarket product or a reading they get from the engine/vehicle body control computer.
The language that says the vehicle's fuel gauge is the marker of record still appears in the U.S. Preferred Service T&Cs:
7.3 The fuel level of the Vehicle at the time You rent it and at the time You return it to Avis is determined by visual inspection by Avis of the Vehicle's fuel gauge.