Apple has stopped signing iOS 13.4 following the release of iOS 13.4.1 last week. This means that, if you’ve upgraded to the latest version of iOS, it’s no longer possible to downgrade again.
iOS 13.4.1 most significant improvement was patching a problem with FaceTime that broke compatibility with older iPhones and iPads. News of the bug came to light shortly after the release of iOS and iPadOS 13.4 on March 24. Thanks to the upgrade, that is no longer a problem.