As Apple publishes newer versions of iOS, it is common practise for Apple to discontinue signing earlier versions of iOS so that iPhone customers are always running the newest version of Apple’s software. In addition to new features and bug fixes, newer versions of each piece of software also come with speed and security upgrades.
Restoring an iPhone now results in just having iOS 15.4.1, since Apple has disabled the ability to go back to iOS 15.4. The newest iOS betas may still be installed by developers and the general public.
If you’re having troubles with a newer version of iOS, you may always downgrade to an older version of iOS to get around them. However, this choice is typically limited. Those who are having problems with iOS 15.4.1 will have to wait till Apple publishes a new version.