Apple would just update you out of existence forcing you to buy another new phone you don't need
I've got an iPhone 7 and just did the iOS 15.7.4 update yesterday. If it's a critical security patch, they've done it for really old version of iOS.
About the security content of iOS 15.7.4 and iPadOS 15.7.4
This document describes the security content of iOS 15.7.4 and iPadOS 15.7.4.
support.apple.com