Maybe a hint?

Back in the day GTK 2 was arguably the standard user interface toolkit in many Linux environments (broadly speaking those aligned with GNOME rather than KDE). It would have seemed like a reasonable choice if you wanted some kind of "enterprise" support, since Red Hat's own installer (Anaconda) used it, as well as many of the bundled desktop applications, even if the big names like Firefox and Libreoffice went their own way.

As I recall it the GTK 3 release was accompanied by a big announcement that it would guarantee backwards compatibility, though only in the future, of course.