Apple is no longer signing iOS 18.4, a little over a week after releasing the iOS 18.4.1 update. Since iOS 18.4 is not signed anymore, iPhone users who have upgraded to iOS 18.4.1 are not able to downgrade to iOS 18.4.

It is not unusual for Apple to stop signing an older version of iOS after releasing an update, and the process keeps ‌iPhone‌ users from installing outdated versions of iOS. "Signing" refers to a server-side verification check that's done when versions of iOS are downloaded onto an ‌iPhone‌. Only software that passes the verification check is able to be installed.

Preventing downgrading ensures that customers have the latest security improvements and are not vulnerable to known attacks or security holes.

The iOS 18.4.1 update addresses an issue with wireless CarPlay and fixes several security updates, including one known to be actively exploited, so it is a good idea to install the software if you have not done so already.

Source: Macrumors