Apple has disabled the ability to downgrade one’s iPhone or iPads using local IPSW firmware files starting with the latest iTunes 188.8.131.52 beta release. This means that when one tries to downgrade their iPhone or iPad using iTunes, it will only end up being restored to the latest available version on Apple’s servers.
So, even if you have a locally saved IPSW firmware file for your iPhone or iPad that is still being signed by Apple, you will not be able to downgrade back to it. For example, while Apple is still signing iOS 12.1.1 – iOS 12.1.2, you will not be able to downgrade your iPhone or iPad to it by manually pointing iTunes to an iOS 12.1.1 – iOS 12.1.2 IPSW firmware file locally stored on your PC.
Apple has disabled the Option + Click menu in iTunes 184.108.40.206 so anyone using the latest iTunes beta on their PC will only be able to restore back to iOS 12.1.3 — the latest version of iOS. If you are on the latest iTunes beta and want to downgrade back to an iOS release that is still being signed by Apple but not the latest, you will have to use iTunes 12.8 or iTunes 220.127.116.11 beta or older.
While one can always roll back to an older iTunes version, this trick will only work for a limited time as future iOS releases will likely end up being incompatible with older iOS releases. This move from Apple will definitely put the iPhone and particularly the jailbreaking community in a fix since they frequently use iTunes to roll back to an older version of iOS on their iPhone or iPad.