Good question - you can run 2019 on more recent OSs with this hack:
See Big Sur prise? Bug Surprise? - #38 by Lars_Brennicke
- Run the Xojo app at least once, so that Gatekeeper is out of the way and you can modify Xojo.
- Quiz Xojo
- Right-click the Xojo app and choose “Show package contents”.
- find the file Contents/Resources/SDKStubs/macosx10.14/System/Library/Frameworks/AppKit.framework/AppKit.tbd
- Open in BBEdit
- Under “reexports”, you’ll see ,/System/Library/PrivateFrameworks/UIFoundation.framework/Versions/A/UIFoundation
- Simply remove this text (including he preceding comma). Don’t delete the closing bracket
- Save the file.
Could I use an older OS? Yes, in theory. In practice, let’s assume it’s so impractical that I’m not willing to, for the sake of this thread.