Some preschoolers do wear diapers to preschool because the preschool age child may not be ready to potty train.
Some children are not ready to potty train until around 4 to 5 years old so it's okay if they are wearing diapers to preschool.
Most preschools will accept a child into preschool when they are wearing diapers and they are usually equipped to change diapers on preschoolers.
I worked in a preschool as a helper and I was the one to change diapers.
I had to change around 8 of the preschoolers diapers and none were potty trained at all.
Eventually though they learned to use the toilet once they reached Kindergarten age.
I used to babysit a kid who didn't use the toilet until he was 8 years old and he also had sensory issues and hated wearing clothes.
So when i was babysitting him he would be in my house wearing nothing but a diaper which was actually kind of cute.
Eventually when he turned 10 he finally potty trained and stopped wearing diapers on his own.
Most kids are out of diapers before 5 years old but it's okay if they are wearing diapers longer as there are diapers made for all ages.