Apple recently released iOS 15.4, and as is traditional, Apple has now stopped signing iOS 15.3.1, making downgrading to that version of iOS no longer possible.
Apple earlier this month released iOS 15.4. iOS 15.4 was a major update that added new emoji characters, Face ID with mask support, and many other features.
The new software can be downloaded on compatible devices over-the-air by going to “Settings” -> “General” -> “Software Update.”
Falling back to an older iOS build is often used by users that jailbreak their iPhones. Restoring an iPhone or iPad to a previous version of iOS can also sometimes be helpful for users who experience significant bugs after upgrading to the latest version of iOS.