Apple Stopped Signing iOS 9.3.1 Firmware Files
In less than 3 weeks after the release iOS 9.3.2, Apple has stopped signing iOS 9.3.1 firmware files, meaning users can no longer downgrade from iOS 9.3.2. You can also confirm this by visiting IPSW.me, a website that provides real-time signing status for Apple's firmware files.
Since the firmware files are unsigned, any attempt by users to downgrade will result in Error 3194. However, Apple is still signing new builds of iOS9.3.1 for some of the older devices, which were released to fix the Activation Lock bug. You also do not have to worry if you have a jailbreak device, as of now the only available jailbreak tool for public is for iOS 9.1.
Apple has a longstanding practice of stop signing firmware files once a newer version is released. This is used to force users to upgrade to latest iOS version thus improving security and reducing fragmentation. This strategy also keeps Jailbreakers at bay, as the newer versions of iOS usually address exploits and vulnerabilities used by jailbreak tools.
same thing expected from Android as well...