Following the release of .5.1 last week, Apple has locked iOS 16.5 signing. This means iPhone users who upgrade to the latest iOS version can no longer downgrade to iOS 16.5.
According to Macrumors, this move by Apple is not surprising as the company typically stops signing older iOS versions a few weeks after releasing a new iOS. This action is seen as part of Apple's effort to help users update to the latest features and ensure device security.
Moreover, in recent years, there have been few reasons for users to downgrade to older iOS versions, especially since they often update iOS directly via OTA rather than through iTunes. Previously, most users downgraded iOS for jailbreaking purposes.
Currently, the latest official iOS version is iOS 16.5.1. Additionally, Apple is also testing for an official release to users this fall.