From what I've read, if Intuit choose to, it could use Xcode 12.1 (released in 20-Oct-2020) which includes SDK 10.15.6 which can produce code targeted for macOS 10.13.6 (= High Sierra). Intuit's choice to use a later version of Xcode which includes a later version of the SDK which can't generate code targeted for macOS 10.13.6 is an (unwise) business decision. A developer finds it easiest to ignore legacy support issues, because they are burden. If I were developing code for myself, I wouldn't bother with legacy support because I wouldn't get any value for taking on that burden. A business that cares about its customers can't afford to be indifferent to legacy support.
