https://wccftech.com/apple-stops-signing-ios-13-5-1-downgrade-from-ios-13-6-no-longer-possible/

Apple released iOS 13.6 last week and now, the company has stopped signing iOS 13.5.1. What this means is that you can no longer downgrade to iOS 13.5.1 from the current latest iOS 13.6. While average users might not have much to do with Apple no signing a specific iOS firmware, it does pose a threat to the jailbreak community as exploits are patched whereby killing their status.

This time around, jailbreak users shouldn't be worried about anything as well. This is due to the fact that currently there are no jailbreak tools like Checkra1n and Unc0ver that support iOS 13.5.1 jailbreak. Henceforth, most users will not find a reason to downgrade to iOS 13.5.1 From iOS 13.6.