In 1994, I know both did. I'm just curious. I always like watching taped promo breaks from people that are posted on Youtube. So I'm wondering who was the first to do it.
Content warnings were used before 1994. What changed was that in 1994 representatives from cable companies met and agreed to a standardized set of warning labels and suggested criteria for applying those warnings. HBO and Cinemax, who are both owned by the same parent company, adopted the system first and they both started using it on June 10, 1994. Since they both implemented it on the same day, the question of who showed the warnings first is just a random and meaningless guess at whichever one happened to have a show begin first after midnight on June 10th.
The standardized, 10-category advisory system will go into effect Friday on HBO and Cinemax (which are jointly owned) and later this month on Showtime and the Movie Channel (which also are jointly owned).