If Linux libraries had that much respect for backward compatibility, there would never had been any demand for Flatpak. Flatpak (and snap) is merely a workaround for the lack of a common "Linux platform" with comprehensive, versioned APIs analogous to the Windows API or Android API. After all, Flatpak essentially provides a way to run a distribution (provided by Flatpak runtimes) inside the host distribution.

I think that was the role of the distro to integrate a common theme for the various versions of gtk and qt. Yes, it’s often duct taped. But apart from freedesktop, we don’t have an org dictating a common API for stuff like how a graphical app interacts with the DE.