Following the release of iOS 13.4 on March 24, Apple has stopped signing iOS 13.3.1, which means downgrading to that version of iOS is no longer possible.
iOS 13.3.1 was a minor update that added a toggle for disabling the U1 chip in Apple's iPhone 11, 11 Pro, and 11 Pro Max.
Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date.
iOS 13.4 is the current publicly available version of iOS, with the update bringing a new Mail toolbar, iCloud Folder Sharing, new Memoji stickers, trackpad support on iPad, and more.
Developers can also download iOS 13.4.5, an upcoming update with bug fix es and a new option to share Apple Music songs on Instagram Stories.

source: https://www.macrumors.com/