Comment by dadoum

GTK update schedule is very slow, and you can run multiple major versions of GTK on the same computer, it's not the right argument. When people says GTK backwards compatibility is bad, they are referring in particular to its breaking changes between minor versions. It was common for themes and apps to break (or work differently) between minor versions of GTK+ 3, as deprecations were sometimes accompanied with the breaking of the deprecated code. (anyway, before Wayland support became important people stuck to GTK+ 2 which was simple, stable, and still supported at the time; and everyone had it installed on their computer alongside GTK+ 3).

Breaking between major versions is annoying (2 to 3, 3 to 4), but for the most part it's renaming work and some slight API modifications, reminiscent of the Python 2 to 3 switch, and it only happened twice since 2000.