Apple has stopped signing iOS 13.5.1 after the release of iOS 13.6 to the public last week. It means that users on later versions of iOS can no longer downgrade to the previous version.
iOS 13.6 includes better software update customisation, Car Key support, Apple News+ Audio and many other minor changes. The update also includes security fixes, so Apple blocks downloads of older versions of iOS to ensure that users remain on the latest builds. It also prevents people from jailbreaking their devices.
iOS 12.4.7 has also been unsigned after the release of iOS 12.4.8 last week, which included security fixes.