Unless you very carefully preserve a stable of old devices with old versions of the OS, it's very challenging to test new software on older operating systems.

Unless you very carefully preserve a stable of old devices with old versions of the OS, it's very challenging to test new software on older operating systems. That's one of my least favorite parts of the ecosystem. If a customer reports a problem on an iOS 12.0.x device, and I didn't go out of my way to keep one of my devices stuck on 12.0.x or to preserve the signatures from an old build, there is no way for me to go get a 12.0 device to troubleshoot their problem (unless I get lucky on eBay or similar) and no way for me to install 12.0 on a device I have sitting in my desk drawer. Apple does not want people sticking to old versions of the OS, and makes it hard for app developers to support those who do.