Apple has notified developers that beginning June 1, 2016, their apps will need to support IPv6-only networking. As this is already supported in Apple's frameworks, most apps won't need to make a change.

Apple to require iPhone and iPad apps to to support IPv6-only networking starting June 1

From Apple:

At WWDC 2015 we announced the transition to IPv6-only network services in iOS 9. Starting June 1, 2016 all apps submitted to the App Store must support IPv6-only networking. Most apps will not require any changes because IPv6 is already supported by NSURLSession and CFNetwork APIs.

If your app uses IPv4-specific APIs or hard-coded IP addresses, you will need to make some changes.

Apple offers a couple of resources for preparing your app to support IPv6-only networking, including a developer document and a WWDC 2015 session.