iOS 12.5.6 was released on a week ago.
iOS 15 will be getting patches even if iOS 14 and 13 don't.
iOS 15 will be getting patches even if iOS 14 and 13 don't.
You're treating the exception (12.5.6) as if it's the rule, but it's not.
https://en.wikipedia.org/wiki/IPhone
C-f for "Supported OS release" and there's a chart showing which devices support which OS. 12 was "sticky" in that it was the last one supporting the 5S and 6. 15 will be similarly sticky given the range of devices (6S/SE(2016)/7) that will be stuck with it and will probably get several more years of security updates as a result.