When I first started writing Flex 2 the latest available jailbreak was for iOS 6. I made the choice to make it compatible with iOS 5 and higher. This choice has made adding features to Flex 2 very difficult. Every change has to be tested against all the iOS versions it supports (iOS 5, iOS 6, iOS 7, iOS 8, iOS 9). None of the new technologies that Apple has come out with (Auto Layout, Swift, etc.) that make my life easier as a developer can be added to Flex 2 because it needs to be compatible with older iOS versions. I consider Flex 2 to be feature complete because of these reasons. Flex 2.5 is my way of taking it out of feature complete mode, and adding new things that the community wants. I won't be supporting older versions of iOS in Flex 2.5, only iOS 9+. Flex 2 will remain working and compatible with older iOS versions.
iOS 9 only.
There's a lot things, like UIStackView, that are in iOS 9 that make it easier for me to develop Flex 2.5. Every device that's iOS 8 compatible can be upgraded to iOS 9, so let's look towards the future and not hold ourselves back.