Have you changed the Deployment Target from prior to iOS 10 to iOS 10/next to iOS 10 before creating an .ipa?. Because deployment target is a fancy way of saying “the minimum version of iOS that your code supports”. Be careful though, because if, for example, you set this to iOS 9.0 but use an API that is only available on iOS 10 without checking first, your app will crash!. The safest thing to do is to test your code on a device running the oldest version of iOS you want to support.
Or am I on the wrong track?. Since your app's features are not supported with iOS 10 or it later version anymore? Hence we need to check all the features from your app and have to be determined those APIs/features that are only available on prior to iOS 10. If possible then need to update those Frameworks/Libraries to iOS 10 standards. Whereas most of the System frameworks as well as 3rd party libraries has been expanding their new version, such as EKEventKit, Amazon SDK, Firebase SDK, Payment Libraries(iZettle, Stripe, Paypal etc), etc. Or we can start on app development from scratch.
If I can get the source code I could choose those APIs/features that are only available on prior to iOS 10 and create a documentation accordingly at least.