IPhone uses nand flash, that means there are sectors where previously deleted data may be present. Does any change take place to those sectors over the years that may render the data unrecoverable?
There is a bit written on NAND flash data retention here:
When a NAND flash device is new from the factory, it can retain data in an unpowered state (at specific temperature conditions) for many years. However, data retention specs are not given for a new device. It is very important to note that published specifications for data retention describe the behavior of the device only at the end of its prescribed service life.
The JEDEC specification for data retention tells us that for enterprise storage devices, data retention at the end of the service life shall be at least three months (stored at 40°C). For SSDs in the client computing market, data retention shall be at least one year after the drive’s service life (assuming it’s stored at 30°C). At the SSD level, this service life is specified in total bytes written, or TBW. For client SSDs, TBW ratings range from tens to hundreds of terabytes (1012 bytes), whereas for enterprise drives, TBW ratings are in the petabyte range (1015 bytes) and higher.