Apple is still signing iOS 12.1.2 and iOS 12.1.1 which means users can easily downgrade to these iOS version’s at any time until Apple stops it. But With the latest iTunes beta 126.96.36.199, Apple is not allowing downgrades to iPhone and iPad owners. So do not install iTunes beta for now..
If you have just downloaded an ipsw firmware of the previous iOS version and installed the iTunes beta version then you will not be able to downgrade back to those iOS. Instead, your iOS device will be loaded with the latest iOS 12.1.3 version.
With the 12.9 iTunes Beta, Apple has disabled an option to select the ipsw file locally from your computer. Now if you hold the Shift/Option & Click the Restore/Update icon on iTunes, it won’t work in this new iTunes beta.
So if you want to downgrade iOS 12.1.3 to iOS 12.1.2 or iOS 12.1.1 then do not install the iTunes beta version. Since Apple is still signing these iOS, you need to hurry up and downgrade back by using an older version of iTunes.