In my case I am a freelancer, so I have strict deadlines and clients might expect me to work the full 8 hours or more. Hell, I've even been asked to work over the weekends and get my hourly rate adjusted accordingly.
I always decline, my flow state does not last more than 5 hours per day with plenty of lazying around on weekends, and working more than that would mean spreading it to a mediocre productivity.
Deadlines are met, and clients are happy (as far as I know).
Unless we are underachievers, I suspect many are in the same boat and would be as productive with 20% fewer hours.