It's a UI pattern that has been superseded by client-side decorations,
apps that used to set the hint have generally moved on to headerbars.
Given that and the limitation to server-side decorated X11 windows,
GTK4 removed the client-side API for setting the hint, it's time to
follow suite and retire the feature.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/221
6e1b14b26b