Confiture
At lower altitude in a light airplane, we don't deal with the effects of air compressibility...so we don't talk about it. We don't consider low altitude subsonic air to be compressible.
This doesn't mean the air ISN'T compressible - it is. It's the same air, just a little denser. Lighties just don't fly fast enough to enter into compressibility effects. We refer to Vne (Velocity Not to be Exceeded) beyond which lies flutter (which is an interplay of airframe rigidity and aerodynamics), rather than Mmo / Vmo. VNE is a fixed number, and TAS related rather than IAS. (or so I have been taught...)
It's still possible to experience 'Mach effects'/compressibility at low altitude, but less likely / common for reasons phsical (*generally* the speed of sound reduces with height due to the reducing temperature), and more practical (there are speed limits in place below 10,000ft, though they can be cancelled).
Beyond that, I'm into guessing: but I suspect the most speed limiting factor down low is economics! The fuel penalty in operating fast 'n' low in all that draggy air is probably the main reason why civil a/c don't.
What I would like to understand is why airliners are able to operate to a Vmo which (I believe) is variable with altitude, and can typically fly higher TAS at altitude - which suggests that flutter isn't limiting at altitude for them? Wheras I understand flutter to be an airstream speed issue, NOT a dynamic pressure issue. Or are they simply not flutter limited at any speed / alt? In which case, what does limit them low down, other than economics?
Quite frankly, the rationale behind which some limitations apply to IAS, and some TAS escapes me - I know which I have to watch for various, but not WHY!