Apple released iOS 13.7 for everyone last week, which includes a major expansion of the COVID-19 Exposure Notification platform. Following this release, Apple today has stopped signing iOS 13.6.1. That means users can no longer downgrade from iOS 13.7 to iOS 13.6.1.
iOS 13.7 lets users enable COVID-19 Exposure Notifications directly in the Settings app on their iPhone. Once you enable exposure notifications in the Settings app, your iPhone will begin monitoring with Bluetooth to log possible exposures so you can be notified of a potential COVID-19 exposure based on the guidelines set by your local public health authority.
Prior to that, iOS 13.6.1 was released in August to fix a known issue related to the Exposure Notifications and another one that could make the displays exhibit a green tint.
Reverting to older iOS builds is common for those with jailbroken devices and can sometimes also be helpful for users who experience significant bugs after upgrading to the latest version of iOS.
Now that Apple has stopped signing iOS 13.6.1, users can no longer downgrade to that version of iOS from a newer version. By preventing users from reinstalling older versions of iOS, Apple ensures that customers are getting the most stable experience currently available.