I think there are also a lot of people that 50 years ago would have felt the same way as now, but would not have had words to express it. I always knew that I wasn't "normal" as regards gender, but I just assumed it meant there was something wrong with me. And when I heard about transgender, I only heard the "woman trapped in a man's body" stuff and the Jerry Springer versions of things, and that didn't fit how I experienced myself.
It was a blog post about 10 years ago that opened my eyes to the possibility that the word "trans" could describe what I had always felt. And websites like Susans helped me learn from trans people all the ways one can be trans, without the distortions and slanders that the more "official" sources claim.
So I suspect that a lot of the people that now identify as non-binary would in pre-internet days either have tried to live as something like lesbian or gay, or maybe binary trans, or maybe just lived in misery (or killed themselves.)
FWIW, a previous trans survey (before the 2022 one) found that roughly 1/3 of the respondents considered themselves non-binary.