Apple Stopped Signing Jailbreakable iOS 12.4, Downgrade No Longer Possible Apple released iOS 12.4.1 on August 26. which is two weeks ago, and momentarily, they have stopped signing iOS 12.4, this implies that iPhone, iPad, and iTouch users who have updated to iOS 12.3 won’t be able to downgrade to iOS 12.4.
By stop signing certain iOS version, Apple is ensuring users to have the latest security experience and prevent them from jailbreaking their devices., in this case, iOS 12.4 is actually jailbreakable due to the bug that Apple unpatched in the update, and led hackers to develop tools like unc0ver and chimera, in which allowed users to jailbreak their iOS devices. However, Apple fixed the bug in iOS 12.4.1.
You can always check ipsw.me to get notified when Apple stops signing a certain version.
iOS 12.4.1 is the last update to the iOS 12 operating system, as Apple will publicly release iOS 13 on September 19th.