"Shock cooling" is an old topic, with a number of religious camps, and very little real data
The best data I am aware of (from a glider towing operation) is that it is a risk only if the CHT is above a certain figure. If one makes an effort to cool the engine down gradually before the event which requires a sudden power reduction, then it is not a risk.
This is consistent with PPL flying school experience - because they fly full-rich most of the time so the cruise CHTs are pretty low, so they rarely crack cylinders.
The magic CHT value is unknown but it is probably about 300-350F. Much above 400F, aluminium starts to weaken and above 450F it is pretty weak.
When I was doing checkrides in my TB20, I would tell the instructor that a simulated engine failure is not to be done unless the engine was cooled first, by crusing at say only 18" and full-rich.
The PPL training practice of briefly increasing power to "warm the engine" is IMHO bollox. If one had a CHT gauge, one would see that the brief power burst do more or less nothing in CHT terms.