So still no GTK3? I think when GTK4 is released in a few months, it would be a good time to move to GTK3 as they can still be outdated. I know I am being a bit bitter for an open source project with less funding, but GIMP was supposed to be a poster boy GTK application in the first place.
At the same time there is an ongoing transition to use GEGL as the pixel manipulation API, which eventually plugins also need to be updated for.
2.10 is the release which provides all the new APIs for plugins to use, so they can start updating (but existing code will work). 3.0 will then remove the old pixel manipulation APIs, and also transition to GTK3+.
This hopefully keeps the burden on plugin devs to a minimum, and keep the compatibility clear for users (plugins for GIMP2 dont work on GIMP3, and vica versa).