Two weeks has been past since iOS 8.2 was released officially. After its release, most iPhone or iPad choose to update to iOS 8.2, but some problems and bugs are found and some people will downgrade from iOS 8.2 to the previous version. Now Apple stopped signing iOS 8.1.3, which means, it is impossible for you to downgrade from iOS 8.2 to iOS 8.1.3.
You can visit this website to confirm this news.
It is not a major issue for jailbreakers as it was not possible to jailbreak iOS 8.1.3. Downgrading can be useful in cases where the previous iOS software update can be jailbroken. So it would have been quite useful if it was possible to downgrade to iOS 8.1.2, 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 over 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, according to iPhone Hacks.