After the release of iOS 15.6 last week, Apple has stopped signing iOS 15.5 on its servers. What this means is that you can no longer downgrade your iPhone to 15.5 any longer.
In practical terms, when Apple stops signing a version of iOS, it means that it is no longer available for consumers to revert back to after an upgrade. The move doesn’t impact your device if you are still running 15.5, but it means that once you upgrade to 15.6, you won’t be able to go back. This is a common practice by Apple as it helps keep devices on the most current version of iOS and makes it easier for Apple to support those devices.
For those who are developers, you will have access to iOS 15.5. The stopping of signing is really aimed at consumers.
As a side note, Apple has also stopped signing iPadOS 15.5 so you are no longer able to downgrade your iPad to that version any longer.
iOS 15.6 brought a number of security updates to the iOS platform and is widely considered to be one of the final releases in the iOS 15 train. iOS 16 is now at Beta 4 and expected to be released laster this fall, perhaps as early as September with the anticipated iPhone 14. iOS 16 will bring a wealth of features and improvements including a completely revamped Lock Screen, improved Mail and Messaging features and other system wide updates.