Apple Won't Make You Upgrade to iOS 15 and Will Continue to Provide Security Updates for iOS 14
Apple plans to allow iPhone and iPad users to remain on iOS 14 and iPadOS 14 even after the launch of the iOS and iPadOS 15 updates, according to new details on the software pages for the new updates.
iOS now offers a choice between two software update versions in the Settings app. You can update to the latest version of iOS 15 as soon as it's released for the latest features and most complete set of security updates. Or continue on iOS 14 and still get important security updates until you're ready to upgrade to the next major version.
With prior updates, Apple has prompted users to update right away and has stopped signing past versions of iOS to make it impossible to downgrade, but it sounds like Apple is planning to relax that policy going forward.
This will be useful for those who are not able to update their devices because of device restrictions or requirements implemented by work organizations or schools.