What follows is the email I sent to Apple's accessibility team regarding my experiences with this issue.
I am writing concerning a problem which I am unable to solve. This issue
concerns an iPhone 6S, 64 GB model. The same problem has affected a
family member who owns an iPhone 6, 128 GB.
Here are the details of my device and the problem I am experiencing.
I am visually impaired and use VoiceOver for access. I use the U.S. Siri
enhanced male voice. My iPhone 6S initially came preinstalled with iOS
9. Its speed and overall performance were extremely satisfactory using
VoiceOver. When iOS 10 was publicly released, I downloaded and installed
it onto my 6S. I noticed that some apps took a longer time to load. I
also noticed a slightly longer than usual delay when performing the
two-finger scrub gesture to move back to the previous screen. While I
wouldn't say that the delays were unacceptable, I admit to being
disappointed at the difference in performance on a phone which is still
a very recent model. I decided to perform a complete reset and erase,
followed by a restore from a recent iTunes backup, hoping that doing so
might increase my phone's performance.
After performing the restore from iTunes, I noticed that there was no
difference in my phone's performance. However, I was unable to switch to
the enhanced Siri voices with VoiceOver as these voices were no longer
listed as available choices. While it was possible to set VoiceOver to
any of the Siri voices, these were just the lower-quality voices. The
enhanced voices no longer appeared as possible choices. Please
understand that these enhanced Siri voices were available to me when I
initially upgraded from iOS 9 to iOS 10. They disappeared when I
performed a reset and restore on my phone.
This is very troubling to me as these are my favorite voices and I very
much prefer them over the other voices from Nuance and I would very much
like them back. As I indicated, another family member who also uses
VoiceOver is experiencing the same problem with his iPhone 6. If there
is a way to return these voices to my phone, I would appreciate learning
how this is done. If this is a bug due to resetting or restoring the
device, I would greatly appreciate it if Apple could investigate the
cause of the problem.