Sadly bigger projects like GIMP or Inkscape didn't learn their lesson. Since version 3.0 GTK serves only GNOME and their particular demands for their severely limited desktop environment. The correct action would be to go back to GTK 2.0. It is far easier to continue to maintain GTK 2.0 than porting your app to a moving target like GTK3/4 with regular severe user and developer hostile regressions.