https://wccftech.com/apple-stops-signing-ios-13-3-as-unc0ver-jailbreaks-public-release-nears-in/

We recently heard that Pwn20wnd's progress on the latest Unc0ver jailbreak is nearing release. While users interested in utilizing the tool can't be more excited, Apple has seen fit to stop signing iOS 13.3. The iPhone-maker might have caught wind of the news that the new jailbreak tool for the new build is right around the corner. The company's latest iOS 13.3.1 is out in the wild now and downgrading to the previous build is no longer possible.

Apple implemented the unsigning of iOS 13.3 firmware a little earlier than its usual timeframe. Probably, Apple might have taken note of the slurry of announcements about the Unc0ver jailbreak for iOS 13.3. Apple might have wanted to contain the situation by disrupting the downgrade process that requires the signing of the older firmware.