Kitchen scraps are any appropriate for the birds to eat...just about anything that is human grade food is fine.
The reason for not leaving food out 24/7 (except for young chicks) is due to problems with feeding rodents, attracting all kinds of other wildlife to the free buffet at night (skunks, raccoons, rats, coyotes, dogs, feral cats) and wild birds during the day (they can harbor infectious diseases transferable to chickens), waste, exposure to wet weather if feed is outside.
I loved the auto feeder as much as anyone. But my feed bill was cut in half once I started feeding the chickens on a regular schedule and not leaving the food around for other animals to eat.