With the release of iOS 13.4 last week, Apple has stopped signing iOS 13.3.1. This means users can not downgrade back to this update and it will no longer be possible.
Apple usually stops signing older iOS versions so they can prevent people from downgrading and so they encourage people to stay up to date.
iOS 13.4 is the latest update for iPhones, iPads, and iPods, and includes iCloud Folder sharing, trackpad support for iPad, new Memoji stickers, and more.
