First I'd like to ask is flash memory used in an iPhone? If yes then please answer my next question.
Flash memory uses wear levelling (changing the physical location on the memory of a logical location) to maximise lifetime. Most flash has some "spare" space that is unreported to allow it to maintain the reported space when an area wears out. This means that there may well be some small amounts of memory that probably holds some unknown data even when the phone is reporting as full. However, it would be arbitrary data and I would expect it would require forensic tools to recover it.
When you reset an iPhone the memory gets encrypted and the keys are deleted. But is the data in the worn out area also encrypted, and does the app shredit and other file shredders also overwrite the worn out area?