I-2021,
You will find that turbopropellor aircraft, whether FAR23 or FAR25, do not have a Vne...only a Vmo (or in some cases) Mmo as well.
This is primarily due to certification limitations...IE: flutter.
Control surface flutter is very bad news indeed, as a few have found out many years ago.
Believe it or not, even such old types as the DC-7 and (later) models of the Lockheed Constellation had Mach number speed limitations.
Lessons were learned the hard way, many years ago.
