Thing is, despite all that trolling about how evil Qt supposedly is, it's the best toolkit under a free license. If Qt was so very evil, why is GTK so bad? One would think everybody would flock to it to make it better…
Xfce is the last mainstream Linux shell that's written using GTK.
GNOME Shell is written with Clutter. So is Cinnamon.
Plasma Workspaces never were written in GTK, Unity uses Nux and will switch to Qt, and now LXDE has been ported to Qt. (OK, maybe you can count MATE as major shell.)
And while I'm aware of GIMP’s roadmap, the GIMP team hasn't really been in a hurry to make the jump to GTK3.