If you look carefully at the design critiera/limitations for some older (but still later designed) piston transports (Lockheed 1649A Constellation, Douglas DC-7 come to mind), you will find a limiting mach number listed in the AFM.
At first this would seem to be a mistake, however there is a reason, and it has to do with compressibility issues discovered during the early testing of experimental aircraft in the early 1950's.
It was not unusual to find, for example, even where a limiting mach number was not listed, a limitation to indicated airspeeds above a certain altitude, typically in the mid-teens, such as (example)...'reduce Vne by 4 knots per 1000 feet above 14,000.'
Turbine aircraft do not have a Vne as such, they have a Vmo which, if it was a piston transport using the same airframe, would have a Vne considerably higher, as that is how the certification standards were at the time.