iOS 10.2.1 and iOS 10.3 can no longer be installed on iPhone and iPad starting last night, Apple blocking the signing of SHSHs for users all over the world. Although it was expected that the installation of iOS 10.2.1 would be blocked by the Apple company now, no one expected that even iOS 10.3 would be banned from signing by the Apple company, iOS 10.3.1 remaining the only one that can be signed.
In general, the Apple company blocks the signing of SHSHs for old versions of iOS 3-4 weeks after a new one is launched, in the case of iOS 10.3 the move was rushed. iOS 10.3 was released only 2 weeks ago by the Apple company, so its installation should have been possible for at least another week, but the Apple company has a completely different strategy.
There is a possibility that the installation of iOS 10.3 was blocked together with that of iOS 10.2.1 because Apple wants to prevent the release of jailbreak solutions for iPhone and iPad. Jailbreak solutions for iOS 10.3 were announced last weeks, so Apple is probably trying to block all potential exploits in order not to allow exploitation of its operating system.
Given the change from last night, you can now only install iOS 10.3.1, or possibly a beta version of iOS 10.3.2, but nothing else. If a jailbreak solution for iOS 10.2.1 or iOS 10.3 will be released and you have updated to iOS 10.3.1 or iOS 10.3.2, then you will never be able to use it and you must be aware of this.
