After the public release of iOS 15.4 last week, Apple stopped signing iOS 15.3.1 for all iPhone, iPad, and iPod touch users. This means users can not downgrade their devices to iOS 15.3.1 version.
It is a common policy of Apple to stop signing older versions of iOS when a new update is released which ensures that users stay on the most updated version of iOS to enjoy new features and are protected by security updates.
It also discourages jailbreaking because users who jailbreak their devices can not immediately upgrade to a new iOS release. Until a jailbreak is available for the latest iOS version, jailbreakers have the option to either stay on the older version or simply update.
Apple no longer signs iOS 15.3.1 version
In February, the company stopped signing iOS 15.3, which prevent users from downgrading their devices from iOS 15.3.1 update that patched a security vulnerability found in WebKit, the engine powering Safari, and fix for a bug that may have caused Braille displays to stop responding.
If an iPhone, iPad, or iPod touch needs to be restored, the device will be upgraded to iOS 15.4 which is currently the latest stable version.
The latest iOS 15.4 update introduces several new features like 37 new emoji including saluting face, biting lip, heart hands and more, support for unlocking with Face ID while wearing a mask, Universal Control is finally available in macOS 12.3 beta and iPadOS 15.4 beta, allows third-party apps to fully utilize 120Hz ProMotion display on iPhone 13 Pro, support for Custom Email Domains via iCloud+ subscription, a ‘Cosmetic Scan’ trade-in tool for iPhone, Apple News app is adding sports scores and favorite team selection, option to disable Shortcuts automation notifications, “Tap to Pay” feature in the U.S., re-enables “Improve Siri & Dictation” for all users, new browsing filter options for Apple Podcasts app, new prompts for iPhone users to “Review Emergency SOS”, ability to save passwords with user name for iCloud Keychain and much more.