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.
I let 21 hens free range and more 15 chickens I keep in a 10x18 room and in total they eat maybe 35lbs of food a week. Plus a 1/2 coffee can a day for my ducks. I use 3 30lb Automatic feeders that are Inside the coop along with 2 3 Gallon waters and 1 5 gallon. I have 1 30lb feeder and a 5 gallon for the group of 15 and the rest for the 21. I have always used auto feeders (except chicks) and I don't really have rodent problems.