Precisely this, the moment they announced they wanted to break backwards compatibility with new GTK versions every 6 months or whatever it was they made it very clear it is exclusively a Gnome API for OSS projects and nothing else.
They backtracked a bit on that but they'll still replace GTK4 with GTK5 at some point, probably deprecating context menus or whatever else this time. Clowns.
The first release of GTK3 that could be considered stable was 3.20. Either way, I'm talking about GTK4, where the original plan was 6 months of breaking releases, stabilization in version 4.6, followed by starting work on the incompatible GTK5, to be released 2 years after the first GTK4 release.
The backlash was so big they at least changed the versioning scheme, but the "break shit every 2 years" is still in full force.
GTK3 did not break its C API. The CSS you could inject was arbitrary and undocumented. They stabilized and documented that in 3.20, yes. I think its very dishonest to say the toolkit was breaking, the vast majority of GTK3 apps didn’t have regressions IME.
They backtracked a bit on that but they'll still replace GTK4 with GTK5 at some point, probably deprecating context menus or whatever else this time. Clowns.