There were a few GT4 cars that were not carried over to GT5:
Audi Auto Union V16 Type C Streamliner
Ford Model T Tourer
FPV F6 Typhoon
FPV GT
Hyundai Accent Rally Car
Mercedes-Benz Benz Patent Motor Wagen
Mercedes-Benz Daimler Motor Carriage
Nike One
Toyota ESSO Ultraflow Supra (JGTC)
Toyota Motor Triathlon Race Car
The Auto Union and Hyundai Accent were brought back for GT6, but still no sign of the others. Just PD logic I guess (or licensing).