Apple stops signing iOS 9.0.2 and from now on downgrade from iOS 9.1 firmware is no longer possible. This happened a little over a week since the release of iOS 9.1. This means that no longer can be restored to the version that Apple released a month ago and close to the ability of users to jailbreak since, at the moment, the latest jailbreak version is only with iOS 9.0.2
For Apple, now we know, it is usual to block the firmware of older versions of its mobile operating system; this with the California company allows users to keep the latest versions possible which are constantly and continually updated to remove bugs, vulnerabilities, and with the introduction of new features.
iOS 9.1 fixed many bugs from version, 9.0.2, certainly not flawless for stability and performance. As we already know Apple is already working on iOS 9.2 which should bring further improvements in performance.