Modern turbo designs (most anything in the last 25 years, at least) use water cooled turbos and convection keeps the water flowing after the engine is turned off. This is why we don't use turbo timers any more.
And also even back then it affected almost no one, there were loads of tests done on this by automotive magazines of the era and found out that usually the last 1-2 minutes of gentle driving to your driveway/garage is more than enough to cool down the turbo properly, the only people affected were those who due to either aggressive driving or circumstances of their location would drive at high rpm, stop, then immediately shut down their engines.
