Never heard of that! They will flower under 14 ,12 is the time you need it depends on the plant, indica, sativa, genetics,the environment, stressors, and the food you feed them on a photoperiod plant Green Kush took about 9 weeks on 12 hrs good luck
No, I would say that is a myth, no discernible benefit whatsoever in my experience
If there was any truth in it, we would all do it as a matter of course; there would be no debate or speculation
People do 8 or 10 hours, 14-16 hours darkness. They finish a little earlier and the yield is a less. Some guy on another forum swears by it. I've done it on some forever flowering sativa's.
Not a myth in any sense of the word, just basic plant biology. People who say it's a myth can never have tried it themselves.