This can be confirmed by visiting ipsw.me, a website that provides real-time signing status for iOS firmware files.
Downgrading is useful in cases where the previous iOS software update can be jailbroken. So it would have been useful currently if it was possible to downgrade to iOS 8.4, which can be jailbroken using TaiG and PPJailbreak. Some users also prefer to downgrade back to the older version if they face problems after upgrading to the latest iOS software update.
This doesn’t come as a surprise as for the last few years, 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, so you can’t use the TaiG jailbreak tool to jailbreak iOS 8.4.1. Pangu team has demonstrated an iOS 8.4.1 jailbreak though it seems unlikely that they will release it to the public with iOS 9 just round the corner.
So given the current situation, jailbreakers should be very careful while installing jailbreak tweaks as a bad tweak could easily force you to restore your device, which would mean that you will lose the jailbreak and all the tweaks installed on your device.