iOS 16.3.1 was released on February 13 and was a minor update with bug fixes, security improvements, and additional Crash Detection optimizations for iPhone 14 models. Apple routinely stops signing older iOS releases over time in order to prevent users from downgrading to an outdated software version.
iOS 16.4 will likely be unsigned later this month, so iPhone users who wish to downgrade to that version have limited time remaining to do so.