Some children have gone back to wearing diapers after they've potty trained but they wouldn't go back to wearing diapers if the parents didn't allow the child to wear diapers again after potty training.
If they don't have access to any diapers to wear then the child won't be able to wear diapers again after potty training.
After potty training you should not allow your child to wear diapers again during the day at least.
If your child can use the potty during the day then they should be out of diapers during the day.
But at night the child may need to wear a diaper to keep their bed dry at night so a diaper at night is okay but you should let the child know that the diaper is for only at night and not during the day.