I was born in 1960. Children with significant difficulties were not in mainstream schools in my childhood. Those who struggled in mainstream at my primary suddenly disappeared (to the 'special school').
At high school (Scotland, the 1970s) children who were deemed not to be coping were moved into the Remedial Department in 'the huts'. In Third Year, they were joined by those who were struggling academically and the 'R' class became the 'S' class - and those children left at 16 with no qualifications.
We definitely had autistic children. One of my schoolmates had an older autistic sister who was kept at home. One of our neighbours had an autistic non-verbal teenage daughter who was left to run around the block and chase after younger children. (The poor soul was just trying to play, but she was adult size and we were frightened of her.)
It's now clear to us that one of my mother's uncles and two of her cousins were 'high-functioning' and on the spectrum.
(Great-Uncle bought a field close to his brother's home. He built his 'hut' with a functioning fireplace. Whenever anyone came to the house, he sought refuge in his hut. His Shetland pony used to curl up in front of the fire.)
When my younger cousin's son was diagnosed with Asperger's, that helped us to understand a great deal about our family.