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.
That's funny. Because I remember one of the changes from gtk-1.2 to gtk-2.0 (or maybe 2 to 3?) was to drop Win32 support. Which I at the time didn't like. We go full circle.
Gtk 1.2 did not really have great Win32 support, was basically a one man show primarily to get gimp to run on Windows. The Win32 backend took off after gtk2, there was more commercial push from Ximian to make it a cross platform toolkit via Gtk# as well. And it wasn’t dropped in 3 or 4, though it definitely has lost development steam as commercial interest in it and cross platform desktop GUI toolkits in general has flagged and consolidated.
I think I had it one major release off. It was probably the gtk3 era. I read comments on the mailing list that you basically shouldn't rely on Win32 support.
You're right that gdk2 ramped up Win32 support, I remember that now.
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.