I would say the passing era, which was a natural progression of the NFL changing the rules to facilitate the passing game.
Irrespective, ultimately it comes down to the QB being able to read the field quickly, no matter the pocket he is operating from, whether drop-back or shot-gun. In five step drops though, one has to be an elite passer to succeed.
Brady, while operating plenty out of shotgun, diagnoses the defense quickly and knows where he going from the beginning. But when he has to go through his progressions, he is successful. He has his weaknesses as well, because he drops to middle of the pack when he gets pressured. He has had great protection since forever, but part of that is also the ability to read what the defense is doing.
I don’t think many QBs of today could be successful in the past, because of how defenses were allowed to play. You had to be a traditional pocket passer that could throw the ball from anywhere, under pressure, make a decisions quickly and take a hit in the process. Even throwing the ball away to not take a bad play is a huge skill, particularly that era.
Romo said it best I think:
“People talk about potential for quarterbacks, and it is one of the most overrated comments. To me, when I look at a young quarterback and a GM asks me, ‘What do you think about his potential?’ I can’t answer until I see how fast he can get through progressions. And when I say that, I mean I need to see if he understands spatial awareness and his ability to go from his third to fourth to fifth even possible guy—and how fast and long that takes him when he doesn’t know the coverage. You can teach someone footwork and teach them how to throw a football but it is very difficult to teach someone how to see things quicker. That’s what separates the quarterbacks who are at the highest level.”
https://www.si.com/mmqb/2015/09/17/tony-romo-dallas-cowboys-week-2-nfl
https://www.si.com/mmqb/2015/09/17/tony-romo-dallas-cowboys-week-2-nfl