In fact, the only change from your characterisation is that the toolkit has been renamed from Gtk+ to GTK. Here is an example of a current claim: "GTK is a library for creating graphical user interfaces. It works on many UNIX-like platforms, Windows, and macOS." https://docs.gtk.org/gtk4/overview.html

I think what has changed is user expecations; in the the 1990s, there was an expectation that programs should look like the native toolkit; whereas by now, it's even questionable whether there is a native toolkit on Windows (due to the range of choice, each of which a noticeably different in look and feel) and program-specific appearance is the norm on mobile and web applications. Nowadays, you could probably write a slightly customised Adwaita app for Windows and you'd get by more satisfactorily than if you wrote a plain GTK app and tried to theme it to modern Win32.