I am certain shock cooling is not a myth, but it is an issue only if the CHT is high enough to start with i.e. when the metal is not at its normal strength.
There is a well known article by a gliding tug operator who reported consistent cracked cylinders, until they did a bit of a low power section before the (rapid) descent, and it cured the problem.
A simple rate of change of temperature is unlikely to be a problem otherwise you would crack cylinders due to shock heating during takeoff.
Lyco recommend 60F per minute max rate of change, which is fair enough. The EDM700 warns if this is exceeded and it never is in normal ops at any significant value of CHT. It is easily exceeded just before landing but by then the engine is cool as a cucumber anyway.
When descending, I reduce MP by a few inches per minute at most.
The ROP v. LOP is no longer an argument, BTW
It is well established.