It’s still possible to generate MPC codes for IAD arrivals. But the days of MPC being a reliable time saver to get ahead of the crowds and/or minimize additional questioning and passport checking time became history when CBP got into the business of wanting to push Airside Mobile out of the scene and to ramp up Simplified (Biometric) Arrival. It’s not unusual for CBP employees even at a place like IAD to sometimes not know or care what to do with MPC users other than to just do the same thing as for the rest of the returning American masses.