Minecraft wiki entry for Superflat_survival, associated text: String : Can be used to make wool which can be used to make beds. Also string can be used to craft bows or fishing rods.
Rotten Flesh : emergency food source.
Raw Chicken : emergency food source.
Feather : can be used to craft arrows
Rabbit Hide : can be used to craft leather. Also it can be used to craft bundles ‌[upcoming].
Rabbit's foot : useless because there is no brewing.
Phantom Membrane : useless because there is no brewing or elytra.