Apple Stops Signing iOS 15.4 Following iOS 15.4.1 Release, Downgrading No Longer Possible
Following the March 31 release of iOS 15.4.1, Apple has stopped signing iOS 15.4, the previously available version of iOS that came out earlier in March. As iOS 15.4 is no longer being signed, it is not possible to downgrade to that version of iOS once you've installed iOS 15.4.1.

Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date, so it is not unusual that iOS 15.4 is no longer being signed.
iOS 15.4 was a major update featuring support for unlocking an iPhone while wearing a mask, new emoji characters, Universal Control on the iPad, and tons more.
iOS 15.4.1 added a fix for a battery drain issue on the iPhone, but more importantly, it addressed AppleAVD issue that could allow an application to execute arbitrary code with kernel privileges. Apple said this exploit may have been actively exploited, so it is a good idea to update to iOS 15.4.1 if you have not done so already.
Apple has also stopped signing tvOS 15.4 and HomePod 15.4 software.
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Top Rated Comments
Once you update your phone to 15.4.1, Xcode 13.2.1 won't allow you to deploy to your phone because it only supports 15.4.0. So you update to Xcode 13.3 but your macOS won't allow it because you're on macOS 11.3 but Xcode 13.3 requires macOS 12.0. So you gota buy a new Macbook.
I took that as a warning sign and leave it be. The watchOS 8.5.1 update failed a couple times, but I just put my S3 next to my X before going to bed and updated the next morning.
So, things are going well otherwise. See you back here next year with iOS 16.
In the company I work at, I also receive email notification about needing to go to the latest version otherwise access to company resource will not work... I don't have a work-provided phone and I just decided to upgrade...
Also, note to Apple, re-sign the last update of previous versions so if someone cannot afford to buy a newer model, the device they currently have will still be at least useable although with limited app support...