Apple will no longer be signing iOS 13.3 after the release of its newest version, iOS 13.3.1, which means that rolling back to iOS 13.3 is no longer supported.
Apple routinely stops signing older OS versions when a new one comes out to encourage its users to update. iOS 13.3 in particular was a major update that had a lot of new features and fixes, including Lightning FIDO2, USB and NFC security keys, Screen Time communication limits and more.
In retrospect, iOS 13.3.1 has added updates such as the Ultra Wideband chip for the iPhone 11 lineup and various improvements and bug fixes. It’s currently available to download and install on supported iPhones.
Public beta testers and developers can try out iOS 13.4, which is currently in beta version. It has features such as new Memoji stickers, iCloud Folder Sharing and a new Mail app toolbar, among others.