Correct me if I'm wrong but it's my understanding that macOS 15 security updates are only made available for devices for which 15 is the last compatible macOS version.
That's not how it's worked in the past. They did security updates until three versions later released regardless of the hardware. What you may be thinking of is that there was some weirdness around Intel vs M series chips and how those got updates.
Parent is thinking iOS/iPadOS, not macOS. Older devices that are not capable of running iOS 26 are still receiving security updates for iOS 18, but any device supported by iOS 26 is not. It's a real hard push by Apple to get people to swallow Liquid Glass by withholding security updates.