Apple is no longer signing iOS 18.0.1, which means you cannot downgrade to this version of Apple's operating system if you have upgraded your iPhone to iOS 18.1. iOS 18.1 was publicly released a week ago, on October 28.

Apple usually stops signing older OS versions shortly after a new version of the OS has been released. This is mainly done to encourage iPhone users to stay up to date with the latest operating system and the security fixes it brings.

When Apple stops signing an update, it can no longer be installed on an iPhone. This is done via a server-side software verification check that happens before new software is installed, and it would not let you install an older version of iOS if you have the latest one.

iOS 18.1 is a very good update and I would personally not see why someone would want to downgrade after installing it. We've had cases in the past where a new iOS version is quite buggy and I would understand why people would want to ditch it in favor of the previous one that worked just fine. But this seems not to be the case this time.

iOS 18.1 adds the first batch of Apple Intelligence features, which include Writing Tools, a new look for Siri, summaries, and smart replies, among others. Tim Cook, Apple's CEO, recently said that iPhone users are upgrading to iOS 18.1 twice as fast as iOS 17.1 during the same time last year.