As an application developer, should I still care about gtk_application_prefers_app_menu () then? Do you know if any desktop out there use such a global app menu, or was it only GNOME?
And finally, does this function still make sense for GTK4?
You can ignore the method entirely if you have redesigned your application to follow the latest GNOME human interface guidelines, and moved the entries from the app menu to a different location inside the window.
It might make sense to remove the method from GTK4; care to file an issue?