Although the NFL and the Players Association have been testing helmets for five years, there's no longer a grandfather clause allowing players to keep their preferred headgear if it loses certification.
"The prohibited helmets will not be allowed in the locker room," Jeff Miller, the NFL's executive vice president of health and safety initiatives, told reporters in April. "A player will not be allowed on the field [wearing such a helmet]."
https://bleacherreport.com/articles/2849093-raiders-rumors-antonio-brown-files-helmet-grievance-against-nfl?utm_source=cnn.com&utm_campaign=editorial&utm_medium=referral