Update November 13, 2017: Using Decipher Backup Repair or Decipher Phone Refresh to downgrade an iOS 11 backup to iOS 10 can be handy if you need to restore a backup to an older device to get data out of an unsupported 32-bit app that won't run on iOS 11. In that case, the issues below about the stock iOS app data not being compatible is not really an issue. If you're interested in doing this, contact us and let us know the situation and we can send you a custom version of Decipher Backup Repair that will include downgrades as part of the repair process.
(Original post from June 6, 2017 below)
Now that the iOS 11 developer preview is out, we've gotten a few emails about restoring a backup made with iOS 11 to a device running iOS 10.3.2. Presumably this is for people who put their personal iPhone on the beta, but now want to switch back to non-beta iOS.
I've tinkered around with “downgrading” an iOS 11 backup so it can restore onto iOS 10 and had some success, however there are three big caveats:
- The Messages app data from iOS 11 is not compatible with the Message app on iOS 10.3.2.
- The Activity/Health data from iOS 11 is also not comptatible with Activity in iOS 10.3.2.
- There is likely more data in the same predicament!
For most people, this makes downgrading a backup not worth the effort, since Messages and Activity are two of the most popular kinds of data needed from an iTunes backup. So the short answer is no, we don't recommend trying to downgrade an iOS 11 backup to restore onto iOS 10.
Hopefully more stable iOS 11 betas will be available soon, especially as the public betas are released, and then this won't be needed!