After releasing iOS 13.1.2 last week, Apple has now stopped signing iOS 13.1 making downgrades to it no longer possible. Just late last week, the company had stopped signing a bunch of iOS releases including iOS 12.4.1, iOS 13.1, and iOS 13.1.1.
As of now, the only iOS 13 release being signed by Apple is iOS 13.1.2 — the very latest public release of the OS. What this means is that you cannot downgrade your iPhone back to a lower version of iOS 13. You can, however, downgrade it back to iOS 12 as the company continues to sign iOS 12.4.2.
Apple rolled out a number of iOS 13 updates within a short span of time after the initial public release. The first iOS 13 firmware was quite buggy which led Apple to do this. iOS 13.1.2 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 of now, there is no jailbreaking solution for iOS 13 but it is usually recommended that you stay on the lowest firmware possible of the latest iOS release if you want to jailbreak your iPhone or iPad. We might see a jailbreak tool for older devices like the iPhone 6s to iPhone X using the checkm8 exploit. In case that happens, the iOS version will likely not matter since its a bootrom exploit that Apple cannot patch via software.