Apple is more and more pushing users to use the latest OS. That they removed the option to hide the nagging notification of new OS versions with a security update was just the start. Just providing a single patch for iOS 14 after the iOS 15 launch after promising 'it would continue to receive update' was a big FU to users.
I'm using a lot of apps that are too complex to be fully compatible with a new OS on day one, so I'm still running Big Sur. I'm not sure why Apple isn't updating Big Sur and Catalina. They really should communicate about it, but Apple's communication is lacking lately. I don't see why they can't backport the fixes with minimal effort certainly if they're actively exploited.