Apple has stopped signing iOS 13.1.1 and all the lower version of iOS 13, that means it is no longer available to downgrade your iPhones to them. It has also stopped signing iOS 12.4.1., the last stable release of iOS 12.
It isn’t possible to downgrade your iPhone running iOS 13 to iOS 12.4.1. Although Apple continues to sign iOS 13.1 for the time being, though it has stopped signing iOS 13.1.1 and iOS 13 itself.
Apple introduces number of updates on iOS 13 within a short period of time after OS is a public release. The iOS 13 was quite buggy so Apple leads to introduce iOS 13.1.2 which is comparatively bug-free and stable in nature so it makes sense for the company to stop signing all previous buggy releases of the OS.
As in the present, there is no jailbreaking solution for iOS 13 but it is prescribed to stay on the lowest firmware possible of the latest iOS release if you want to jailbreak your iPhone or iPad.
We can have jailbreak tools for the for older devices from iPhone 6S to iPhone X using checkm8 exploit.