For some reason they patched one and only one actively exploited bug in iOS 12. They did not patch the bug in iOS 13 or iOS 14. iOS 14 hasn't received any security updates since October 2021. It's been even longer for iOS 13. And also for iOS 12, except for that one unusual patch. https://support.apple.com/en-us/HT201222
We can't treat the exception as the rule. It would be misleading to claim that Apple is still supporting iOS 12, or 13, or 14. Every other security patch in iOS 15 has been left vulnerable in pre-15 versions.
iOS 12 was the last release for the 6, which was the last phone cut off from updates (and they sold it for an especially long time). The 6s and 7 can update to iOS 15, which is still getting security updates.
We can't treat the exception as the rule. It would be misleading to claim that Apple is still supporting iOS 12, or 13, or 14. Every other security patch in iOS 15 has been left vulnerable in pre-15 versions.