Apple has stopped signing iOS 15.7, rolling back from iOS 15.7.1 is no longer possible. After stopping signing iOS 16.0.3 this week, Apple stopped signing iOS and iPadOS 15.7. The company previously released iOS and iPadOS 15.7.1 with bug and security fixes for iPhone and iPad.
How can a Mac mini become a MacBook?
The firmware signing process allows Apple to prevent iPhone and iPad users from downgrading iOS or upgrading to older firmware versions that contain security vulnerabilities. Usually these vulnerabilities are used for jailbreak or hackers with bad intentions, but Apple is against both.
Therefore, when the firmware is not signed, you cannot update to it via Finder or iTunes by downloading the IPSW file, as this will cause an error. There are also third-party methods for installing unsigned firmware, such as futurerestore for a rollback and DelayOTA for an update.
In addition to jailbreaking, installing older firmware versions can be useful for those who have encountered any bugs in the new version of iOS and would like to roll back to a previous version that does not have this bug.
If you want to know which versions of iOS or iPadOS are currently signed for your device, you can find this information on the IPSW.GURU website.