For what it's worth, a quick check with glade 3.8.6 found perfect compliance up to and including GTK 2.16:
You have to dial back to 2.14 before anything comes up:
To reproduce:
1. Fetch a glade-3.8.6 tarball from [here](https://download.gnome.org/sources/glade3/3.8/)
2. Install with:
~~~ sudo apt install libgtk2.0-dev libxml2-dev itstool
./configure --prefix=/usr/local \ --enable-debug=yes \ --enable-maintainer-mode \ --disable-gtk-doc-html
make -j4 && sudo make install ~~~
3. Check out 90c6096ed6ea167f9100ce8f74229a3f47acc29a and open `data/geany.glade`
4. Go to Edit > Preferences and run the validator for toolkit version 2.14 (anything later should pass)
5. Check the diagnostics against this list: [GTK+-2.14.deprecations.log.txt](https://github.com/geany/geany/files/6966154/GTK%2B-2.14.deprecations.log.tx...)