Apple has stopped signing iOS 18.6.2, which means that if you've upgraded to iOS 26, you're no longer able to downgrade to a version of iOS 18. Apple released iOS 26 a week ago.

When software is "signed," it means that it has passed a server-side verification check that Apple performs when a new version of iOS is downloaded on an iPhone. An update can't be installed unless it passes the verification check.

Unsigning software prevents Apple customers from installing outdated, less secure versions of iOS.

Apple isn't forcing users to install ‌iOS 26‌, and there is an option to continue using iOS 18, but once a device has been upgraded to ‌iOS 26‌, there's no going back.

Apple has also stopped signing iPadOS 18.6.2 and tvOS 18.6, so there's also no downgrade option available for iPads.

Source: Macrumors