Apple is ending subscriptions for iOS 8.4.1 and iOS 9.0 with the release of update 9.0.2 yesterday.
Thus,
This is confirmed by the ipsw.me website, which monitors the signed versions of the firmware in real time.
Rollback to old firmware is often used tojailbreak. Jailbrokers could, if there was such an opportunity, roll back to version 8.4, which can be cracked using TaiG and PPJailbreak. However, for iOS 8.4.1 there is no jailbreak, so there was no point in rolling back to it.
Some users have returned to iOS 8.4.1, having encountered performance issues after upgrading to iOS 9.
Apple stops subscribing older versions forfight against jelbreakers. The company not only releases new firmware versions that eliminate the system vulnerabilities used for hacking, but also stops the subscription of old firmware for which such vulnerabilities can be found.
In iOS 8.4 firmware.1 Apple fixed several exploits previously used in the TaiG jailbreak; the same thing was done in version iOS 9, so TaiG is not applicable for hacking iOS 9. Pangu jailbreak iOS 8.4.1, but they are now unlikely to release it, since iOS 8.4.1 subscription has been completed.