@Barnbrack
I think you are confusing two different issues here.
There have always been severely disabled children who were unable to toilet train. They might have been in seen less or almost never in mainstream schools up until a relatively short time ago, but there hasn’t been an increase in severe disability.
The rise in diagnosis of neurodiversity and SEN, doesn’t mean that the proportion of those children who are incapable of toilet training before school age has risen. Why would it?
I have already posted about this but in countries where toilet training is socially expected by 3, this is what happens, even if these children receive diagnosis further along the line, that doesn’t stop them from being toilet trained before starting nursery.
I know this because I have had 20+ years working in a county where there is free nursery school from 3-6 and there are no separate schools for children with disabilities.
In an intake of say 50 3 year olds there may be 1 or 2 who come with nappies because they have a clear disability which, even if not diagnosed officially is on the pathway and has been already monitored by a paediatrician. There will be other children who receive a diagnosis of SEN during their time at nursery, but they still start out of nappies at 3!
I say this knowing all too well about severe disability and how for some children this will never be possible. It however has nothing to do with the increase in children coming to school in nappies.
In the schools here there are wet rooms fitted with baths and shower and spaces to change mainly children with disabilities, but there are official roles within the school system where bathing and cleaning is required, the same as when children are physically sick etc.