Apple has stopped signing iOS 15.2.1 to its servers. This means that it is no longer possible to install or restore to this specifc version, whether on iPhone or iPad.
End of signing for iOS 15.2.1
iOS 15.2.1 was released on January 12. This was a minor update that fixed three bugs. The first was that the Messages app might not load photos sent using an iCloud link. It is now history, the photos appear well. The second bug fixed was that third-party apps with CarPlay might not react to user interactions. And the third bug was the one that affected HomeKit and could crash iPhones and iPads.
Now users can install only one version: iOS 15.3. This is the latest version that was released on January 26th. This is again a version that has fixed bugs and made general improvements, without Apple giving specific details.
The next version will be iOS 15.4, it is currently available in beta for developers and for public testers. This version includes several new features, including the ability to use Face ID with a mask and support for Universal Control. Apple does not yet say when the final version will be available for download.