You are correct, today it is specialization, drivers do not race much outside their chosen discipline.
I marvel at someone like Mario Andretti who had success in everything from midgets to Formula 1. Surtees went from world championships in motorcycles to one in Formula 1. Racing yes, but entirely different. It is simply amazing.
May he Rest In Peace.
As a longtime auto racing fan, I have to say that most of my all-time favorite drivers are those from many years ago who drove pretty much whatever was put in front of them. It should be said that for a lot of them, back in those days it was what they had to do to make a living as drivers.