If you wake up one morning and find that you have a burning desire to install iOS 16.3.1 on your iPhone, you're going to be out of luck. That's because Apple is now actively preventing people from doing exactly that.
Apple has reportedly stopped signing iOS 16.3.1 which means that it cannot be installed on iPhones anymore. That's the same whether people want to upgrade from an older version of iOS or want to downgrade from something newer - no matter what, iOS 16.3.1 is off the table as of right now.
That in turn means that the only versions of iOS that can now be installed on an iPHone are iOS 16.4 and iOS 16.4.1. We'd suggest staying clear of iOS 16.4 because while it did include some improvements, it also managed to cause some people to have to re-enter their Wi-Fi password repeatedly. There were also reports of other issues relating to things like the Home app and smart home accessories simply not responding.
Apple often stops signing versions of iOS for a couple of reasons. The first is so that it ca ensure that people can't install an older version to take advantage of a security flaw, which is important for obvious reasons. The second reason is to make sure that a many people as possible have access to the very latest APIs and features to allow apps to take advantage of them. That also allows developers of apps distributed via the App Store to more easily assume people have specific versions of iOS installed.
Considering that iOS 16.4.1 was released recently to fix the bugs added by iOS 16.4, it's surely only a matter of time before iOS 16.4 is no longer signed as well.
Apple is already testing the next version of iOS, with iOS 16.5 now in the hands of developers as well as people who are on the public beta program. Apple will also soon unveil iOS 17, a much bigger software update, during the WWDC23 opening keynote on June 5.
If Apple follows the kind of release cadence that we're used to then we can expect months of betas through the summer before iOS 17 is finally made available to the public in or around September. Apple will also release big new software updates for the iPad, Mac, Apple Watch, and Apple TV at the same time.
That iOS 17 update will of course be the one that ships pre-installed on the iPhone 15 lineup that is itself thought to be set for a September unveiling.