They wouldn't have been in mainstream schools to be obvious in society.
In the early 90s, my friend used hearing aids and was the first hearing impaired student in the school. Other children with disabilities began to be admitted into mainstream around the same time.
ADHD and autism were less recognised but with hindsight there were a comparable number of young people with these traits, and I know some of my friends finally being diagnosed in their 40s.
Struggles were more hidden. No one looked into why people "bunked off", the stakes were lower, less exam pressure, more entry level jobs to work up from the bottom. Arguably the changes in society such as formal childcare, and more intense environments also tips people beyond fragile coping thresholds.
Higher parental age and increased survival rates in premature babies and difficult pregnancies/ births has also increased rates.
Special schools have higher rates of children with complex medical needs so children struggling with the curriculum are pushed into mainstream schools.
Cuts have meant fewer early interventions so more children are starting schooling behind in skills like speech and communication and able to keep up. Waiting lists and processes take years, and children can't develop to their potential in that lost time.
There is a combination of more children, more recognition, and less support/ provision.