This can be used to mark entire types as deprecated, and trigger a warning when they are instantiated and G_ENABLE_DIAGNOSTIC=1 is set in the environment.
There's currently no convenient macros for defining types with the new flag, but you can do:
_G_DEFINE_TYPE_EXTENDED_BEGIN (GtkAppChooserWidget, gtk_app_chooser_widget, GTK_TYPE_WIDGET, G_TYPE_FLAG_DEPRECATED) ... _G_DEFINE_TYPE_EXTENDED_END ()