Good grief. This is extraordinary and really worrying.
I was listening to the Born in Bradford programmes on Radio 4 recently and a teacher there was talking about how far behind many children are when they arrive at school - not toilet trained, struggling with basic social skills, very poor speech and language. She was speaking factually, not judgementally, and didn't go into detail about why this might be happening, but clearly something's gone very wrong in the support and guidance given to parents, and children are being let down as a result. It can't all be down to undiagnosed disability.
My children are in their early 30s now. Obviously there may have been children in their classes wearing nappies or pullups and I wouldn't have known, but I was in school a lot helping with reading and the PTA and I was a school governor. If the school staff had been noticing a trend of more children arriving in Reception who weren't toilet trained I would have heard about it one way or another. I know it was rare for children to turn up at our nursery school who weren't toilet trained from their reaction to my son's occasional accidents (bowel).
Thinking about what might have changed between the 1990s and now, far more families have both parents working. There are probably more single parent families too. However, I'd be surprised if it's working parents whose children have never even started toilet training. No reputable child carer (childminder, nursery or nanny) is going to just ignore the need for toilet training, surely, even if the parents have?
It's not disposable nappies and pullups, as those were available and affordable in the 90s, and the vast majority of parents I knew used them. Those probably did lead to a delay in starting toilet training - more like 3 than 2. My mother definitely thought I was leaving it a bit late, but she would have been desperate to get us out of cloth nappies because of the horror of washing them without an automatic washing machine.
My health visitor was fairly useless, but at least we had one. I have the impression there's far less support for parents now than there was, probably because of the massive cuts from 2010 on. False economy, as it's led to these serious problems which are now going to cost far more than health visitors, subsidised baby and toddler groups and Sure Start would have done.
My hunch would be that the parents whose children are arriving untrained are either not on Mumsnet at all or are studiously avoiding threads like this.