Imagine the privacy violations of refurbished iPhones. You buy an iPhone or get a trade in that’s a refurb and you suddenly have randoms “nudes” appearing in your recents? This can’t be right….
The issue in TFA is probably different. If files are only marked for deletion and their data never overwritten, it's conceivable they might show up again after a buggy update.
Of course, one would assume iOS would actually delete the data of files marked for deletion after some expiry, but it's also possible this wasn't happening.
Overwrite with zeros is generally not done as default in flash memories to improve disk life, flash memories have a limited number of times they can be written too, so most memory controllers will try to distribute writes
This seems less a OS bug, more likely iOS/iCloud had a bug on marking deletion correctly
Two potential issues when relying on flash erasure for privacy, neither of which apply to a full device wipe:
1. Filesystem allocation units are almost always smaller than flash erase blocks; erasing and rewriting, e.g., a 1MB flash block whenever a single 4 kB allocation unit it contains is freed will lead to performance and endurance problems.
2. On desktop and laptop systems, erase (ATA TRIM/SCSI UNMAP/NVMe Deallocate) operations are typically deferred to a low-priority or periodic background task for performance reasons; the same may apply to iOS devices.
In the case of full device wipes on modern Apple devices (iOS and Apple Silicon Mac), this is mostly moot anyway from a privacy perspective, because encrypted storage makes data effectively unreadable as soon as encryption keys are wiped.
Either way, horrible day for Apple's "Privacy First" charade.