In my experience and from talking to my colleagues, it's clear that there has been a big increase in the past decade of children coming to school in nappies. We now have a school policy on it, it's so frequent.
And it's not just toilet training. One of the reasons out lovely 20 year experienced Reception teacher is leaving is the fact that she now has to manage things like toilet training, dressing skills and basic skills like teaching to share, putting shoes on in class.
I personally think it's a combination of busy parents, different child led attitudes and assumption that the school will do it.
I of course accept that children with needs will be different.
Other teachers have mentioned toilet policies. Kids asking to go to the loo is hugely disruptive. In Ks1 it's part of the job to teach them to go at appropriate times. I'm in year 6 this year, and they still ask.to go all the time. Especially if it's fractions. I'm fairly relaxed and ks1 is more so, but children do need to learn bladder control.
I know which kids genuinely need to go when they ask, because I have a good parent teacher thing going on. They'll tell me if the kid has a uti or need.
As for training, I have one asd ds who was done at 2 and one madam who delayed till she was nearly 4. She is August born. We trained intensively using a reward system over the holidays. When the teacher came for home visit, I primed her to give Dd a pack of "school pants". Seemed to work. She wanted to do it after that.