No, do you have articles, pictures, videos, etc. proving that pitchers didn't do it?
I do know that we threw the ball as far as we could from time to time well before 1980. We didn't do it on a regular basis and we didn't call it long toss.
I tried to play some long toss myself on Sunday. I could only throw it about 185'. Then we went to the MLB fanfest. I figured I'd be able to hit about 65 mph on the guns they had there as a result. They had the whole thing set up very poorly making it very difficult to get a reading when trying to throw hard but I did manage to hit 65.
They had non-baseball people running it and they didn't realize they had their Stalkers set up way too close to where people were throwing from. Release points tended to be outside the beam divergence angle they were so close. It worked fine for the little kids but the bigger kids who were throwing fairly hard tended not to get a reading at all.