Apple today stopped signing the iOS 17.2.1 update, preventing iPhone users from downgrading to that version of iOS going forward. iOS 17.2.1 is no longer being signed following the January 22 release of iOS 17.3, which introduced Stolen Device Protection for the iPhone and other changes.
It is not unusual that iOS 17.2.1 is no longer being signed. Apple routinely stops signing older versions of iOS after new releases in order to encourage customers to keep their operating systems up to date and to prevent users from downgrading to older, less secure versions of the iPhone operating system.
Along with iOS 17.2.1, Apple has stopped signing iPadOS 17.2 for the iPad, and the older 15.8 and 16.7.4 versions of iOS and iPadOS for devices unable to run iOS 17.
Apple is already testing iOS 17.4, a major update that is set to come out in March.
Source: Macrumors