This is of course a lie. They declare widely used APIs as "internal interface" and break backwards compatibility on a regular basis even on sub minor version updates.
I think you are talking about the styling API during the 3.10 - 3.16 years.
Yes they did but the styling was never an official API. If you can't find the documentation in Devhelp, then it's nothing you should use.
For people who thought they are smart its of course a fuckery.
The most painfull came from breaking the drawing code: Gtk2 moved to Pango/Cairo, Gtk4 moved to GPU Textures. Gtk5 will now break ageing view models. And i hope Gtk6 will break the single threading of the whole Gui.
