Several factors to consider with modern turbos. One is that most all modern turbos are water cooled nowadays, both gas and diesel turbos. Second, engine oils are better at resisting coking than years ago.
But lets look at a this one. I have a 2000 MY Detroit Diesel Series 60 12.7L. Before the motor was dropped in my semi truck, we replaced the factory waste gate turbo with a Borg Warner 171702 assymetric turbo. It is not water cooled. I have a pyrometer to let me know the temps at the exit of the turbo. The only time I put forth any time for cool down is on extremely hot days when moving heavy loads where the motor is working pretty hard. And then, maybe 2-3 minutes. For all other times, I rarely ever concentrate on cool down. The motor now has 1,019,840 miles on it, it is all original except the water pump. Same turbo, same injectors, etc. Only uses, at most, about 1 quart of oil in 11-12,000 miles. There is no evidence that following dedicated cool down procedures would have delivered any better results.
It's not a bad idea to follow a cool down regimen. But, probably only justified in limited circumstances.
