A week ago, Apple released the final version of iOS 11.3 for all compatible iPhone and iPads. Following its release, the company has now stopped signing iOS 11.2.6 which means that once you upgrade to iOS 11.3, there is no way you will be able to downgrade back to iOS 11.2.6.
iOS 11.3 comes with plenty of new features, enhancements, and bug fixes, so its unlikely that vast majority of iOS users would want to downgrade back to iOS 11.2.6. However, people who prefer jailbreaking their iPhone usually stick to an older release as the chances of a jailbreak being released for it are higher. Given the lack of a jailbreak for iOS 11.2, it is unlikely though that we will see a jailbreak for iOS 11.2.6 being released anytime soon as well.
Currently, it is possible to jailbreak iOS 11.1.2 or lower using Electra Jailbreak or LiberiOS jailbreak. Security researchers have successfully jailbroken iPhone X on iOS 11.2.1, but they have said that they don’t plan to release a jailbreak. So it remains to be seen if someone in the jailbreak community is able to discover the exploits and release the much-awaited iOS 11 jailbreak. If you’re on iOS 11.1.2 or lower then it is always advisable to avoid updating to the latest iOS version, to increase your chances of a jailbreak.