Hey @meghna_810,
Thanks for posting this question! This is a great one as the iOS ecosystem becomes more complex. I will share what I know, but hopefully others have more information as well!
So in general, you will want to compare the SupportedDevices list of the two OS releases. At a high level, the 14.8 release applies to 71 hardware models where the 15.0.2 release applies to 79 hardware models (makes sense as newer hardware has been released that never had iOS 14 installed).
Historically, this gap would exist with major OS releases to indicate hardware no longer supported for major releases as I image you know.
Additionally, now that we have iOS update delays for managed devices, those also need to be accounted for when doing security updates. As an example, an MDM device has the option to now delay a major OS updates, but they can still install security updates without accepting the major OS update. Apple will continue to make security updates to legacy OS versions until they are formally no longer supported and eventually you will see those fall off.
Some information about each OS release can be learned based off of the order in which they are released and also the hardware it is being made available to. The best place to find information on this is Apple Security Page and sign up for the security notifications. The release notes are not always up to date but the security updates will always be up to date from my experience.
https://developer.apple.com/documentation/ios-ipados-release-notes
https://lists.apple.com/mailman/listinfo/security-announce/
More information on the certificates that Apple manages and has in use can be found at the below URL.
https://www.apple.com/certificateauthority/
There are also typically announcements when developer related certificates are changed such as the Apple Push Notification Service certificates.
Hopefully this helps!
