I seem to remember reading that there was almost no autism before the late sixties. Even then, metal absorption was suspect.
No, it has always been around. It wasn’t diagnosed as Autism. I knew some growing up. Some were called Idiot Savants. There was one in my wife’s hometown that sold art he painted from a wagon downtown.
They generally were confined to asylums. My aunt worked in one (St Albans). I remember in college there was an old guy in a nursing home that played the piano while we sang to them.