For the first time, Apple will allow users not to update to iOS 15 while still being able to continue installing the security updates that will also be released for iOS 14.
iOS has always had high adoption rates among users, but some people prefer not to update immediately to understand if the first versions of a major update have more or less important bugs. To date, Apple has released security updates for older versions of iOS only for those devices that could not update to the latest version of the operating system. Users who owned newer iPhones were required to update to the latest version of iOS in order to get the security update.
With iOS 15 things will change, since Apple plans to release security updates for iOS 14 as well, so as not to force users to update to the latest version to keep their devices safe. Apple is likely to require everyone to migrate to iOS 15 only when iOS 16 is released in 2022.