Apple recently released iOS 16.6.1, and as is traditional, has now stopped signing iOS 16.6, making downgrading to that version of iOS no longer possible.
iOS 16.6.1 is a release that addresses a zero-day security vulnerability that Apple says was actively exploited in the wild. If you have not updated, you should immediately do so.
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 who 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.