Downgrading is useful in cases where the previous iOS software update can be jailbroken. So it would have been useful if it was possible to downgrade to iOS 8.4, which can be jailbroken using TaiG and PPJailbreak. But since iOS 8.4.1 cannot be jailbroken, downgrading wasn’t a viable option for jailbreakers. Some users also may want to downgrade back to iOS 8.4.1 after they faced issues such as performance problems after upgrading to iOS 9.
Apple has used this strategy to keep jailbreakers at bay. It not only releases new software updates, that fixes the vulnerabilities used by the jailbreaks, it also stops signing the older iOS firmware file, which prevents users from downgrading back to the older iOS version that can be jailbroken.
Apple has patched several exploits used by the TaiG jailbreak in iOS 8.4.1 and also not more in iOS 9, so you can’t use the TaiG jailbreak tool to jailbreak iOS 9. Pangu team had demonstrated an iOS 8.4.1 jailbreak. There were some hopes the Pangu team will release the jailbreak for iOS 8.4.1, but it seems unlikely they will release it, now that the iOS 8.4.1 signing window has closed.
Have you been waiting for iOS 8.4.1 jailbreak? Do you plan to upgrade to iOS 9?