Today, Apple has stopped signing iOS 13.5 now that iOS 13.5.1 has been out for about a week. But, Apple usually waits a little longer to block downgrades of iOS and iPadOS.

This time, the most obvious reason to bring forward the removal of the signing is the jailbreak which was released. Pwn20wnd released a new unc0ver jailbreak for iOS 13.5 and iPadOS 13.5 so in Apple blocking any upgrades or downgrades renders some users unable to jailbreak.