Indeed, you did not meet many institutionalized people when you were younger, and the ones that weren't institutionalized had to pass as neurotypical with all their might to stay that way.
I'm actually glad it's reasonably safe to have a diagnosis now.
In the early 80s, my parents & I lived in a house. The next door neighbors had an LFA teenage boy, whom we obviously knew about, because we could see him running around in the garden, and could hear some random screaming every once in a while. I know that's the first time I ever heard the word autism, and I am very aware that this situation (ours) was not common: that family couldn't really hide that from their next door neighbors, but it's definitely something that their other 2 kids weren't allowed to talk about at school. No other children were ever invited to this house, and I honestly doubt that anybody but us knew about that son.
When I got to junior high school, I had this amazing, probably Aspie teacher who also had a son hardly anyone knew about --turns out, the kid was LFA as well, but he was in an institution somewhere because they couldn't keep up with his needs.
I'm just saying that even in the last 60 years, there were most likely quite a few low-functioning autistic children, but hardly anyone would know about them outside of the immediate family because they could be hidden away, just like the teenage girl who got pregnant could easily be sent away to a convent until things "came back to normal". What I'm not sure is how many of these autistic children made it into adulthood, because the concern for psychiatric patients a few decades ago was not, umm, not exactly there, and I wouldn't be surprised if I were told that a significant number ended up being so neglected or mistreated that they died.
Now, high-functioning and Aspies: they mostly "pass" for neurotypicals, simple as that. So their traits may or may not be noticed. Now, if you think about the large number of eccentrics over the years, a lot of them were probably Aspies. Every town had its crazy old man who rambled on & on about some super specific topic.
And yes, up until recently, autistic people were more likely to get a diagnosis for some kind of mental illness than for autism, so anyone, including them, would only know of their erroneous label.
Also, aside from the increased awareness and access to diagnosis, I think the fact that the pace of societal change has picked up tremendously played a part, that and technology: if I'm comparing with someone born 40 years earlier than I was, there are aspects of life where I'm better off (e.g. no-one is expected to spend their entire life in the same job, at the same company, now, although changing jobs too often is still frowned upon and perceived as lack of stability/reliability, or I'm now able to do research on autism & find a specialist in a way that would never have been possible 30 years ago), but there are also aspects where I feel that we're bound to have it worse & worse as the years go by (e.g. the constant multitasking, open space offices in professional settings, or social media which have taken gossip, judgment & bullying to previously unheard of levels). So, things that could have been red flags decades ago no longer are, while things that are red flags now simply didn't exist back then.
My point is, as the needs of society push for always more social, always faster, always more simultaneous things, while our brain connectivity still doesn't allow for extensive multitasking, need for social connections & so on, we are bound to see more & more of a clash between Aspie abilities and society requirements, leading more people to realize something is utterly 'wrong' and not working... and to seek diagnosis when they come across resources on Asperger's due to the increased awareness AND increased information technology.
