iOS 15 hasn't been the most famous update from Apple. Recent adoption numbers show that iOS 15 is only on 72% of iPhone models from the last four years, which by Apple's standards, isn't a lot. One of the reasons why people might have not updated to the latest OS is because the Cupertino giant allowed users to install the latest security patches on iOS 14 without updating to iOS 15.
With iOS 15.2.1, Apple removed that option. The company no longer allows users to stay on iOS 14 and install security updates. Instead, the Cupertino giant has reverted back to the original method of updates wherein users are required to install the latest iOS update (iOS 15 at this time) to get the latest features and the latest security updates.
The company has now told Ars Technica that the option to stay on iOS 14 and avoid the iOS 15 upgrade was always meant to be temporary. The company says that it's not a bug that security patches are no longer visible on the iOS 14 software update and adds that the support for security updates has essentially ended.