iOS apps must do IPv6
All new iOS 9 apps must support IPv6-only networking as of June 1, 2016.
It should come as no surprise, since the transition has been in train since WWDC 2015, but Cupertino has announced the cutover date.
It should also be not-too-inconvenient, Apple's post notes, because the key NSURLSession and CFNetwork APIs already support connections using only IPv6.
Developers who use Ipv4-specific APIs or static IP addresses will have to make some changes, and Apple points to a tutorial here.
There's also a Safari-only video from WWDC 2015 here. ®