Apple has stopped signing iOS 18.3, preventing iPhone owners who have upgraded to iOS 18.3.1 from downgrading to that version of iOS. Apple released iOS 18.3.1 a week ago, two weeks after the company released iOS 18.3.

Apple often stops signing an older version of iOS, usually within a week or two after a new version is released. When an update is no longer signed, it can't be installed on an ‌iPhone‌ due to a server-side software verification check.

This policy encourages users to keep their operating systems up to date. By preventing customers from downgrading to older versions of iOS, Apple ensures that devices have the newest security improvements.

Apple's iOS 18.3.1 update included bug fixes and security updates. One of those fixes addressed a vulnerability that was actively exploited in the wild, demonstrating why it is important to keep iOS software up to date.

Source: Macrumors