You don’t need to go that far throughout history, children with severe mental or physical disabilities didn’t live long even half a century ago even in most developed nations they would be abandoned and for those that weren’t society didn’t had the facilities to support them.
Down and autism today seems so much more common simply because these individuals get the chance to live to and through adulthood.
These conditions don’t only affect people’s mental abilities they have a plethora of other health issues too.
75% of people with Down syndrome that die die from respiratory and cardiovascular illnesses as they suffer from immune system issues and frequent inflammation, with the current pandemic a lot of them have died because if they do get infected they are severely affected.
https://err.ersjournals.com/content/26/143/160098
Before modern medicine and the flu jab many of them wouldn’t survive a harsh flu season...
Heck before we knew what Down syndrome was most of them probably perished as infants from infections that healthy individuals would just shrug off.