Not sure why you are so downvoted, because indeed Apple only does full security updates for the very newest (now 26): https://arstechnica.com/gadgets/2022/10/apple-clarifies-secu...
After reading that article where it is apparent that Apple has intentionally used terms that sound similar to obscure what the customer is actually gaining when they upgrade versus update and they intentionally omit the part about older devices not getting all the security updates that are pushed in the updates. I now have some clarity.
I can focus on moving to Linux and in time will be ditching the iPhone. Should've done this years ago.
Ars Technica, a clickbait aggregator whom should have been banned from this site long ago, is hardly a reliable source.
https://www.intego.com/mac-security-blog/apples-poor-patchin...
> Note: Because of dependency on architecture and system changes to any current version of Apple operating systems (for example, macOS 26, iOS 26, and so on), not all known security issues are addressed in previous versions (for example, macOS 15, iOS 18, and so on).