Apple today stopped signing iOS 17.5, preventing iPhone users from downgrading to that version of iOS. The update is no longer being signed after the May 20 release of iOS 17.5.1.

It is not unusual for Apple to stop signing an older version of iOS, and it typically happens a week or two after a new version of iOS comes out. When an update is no longer signed, it means it is no longer able to be installed on an ‌iPhone‌ per a server side software verification check.

Apple prevents users from installing older versions of iOS to encourage customers to keep their operating systems up to date with the latest security improvements.

iOS 17.5.1 addressed a corruption bug that could cause deleted photos to reappear on an iPhone. The update is now the only publicly released version of iOS available.

Source: Macrumors