not usable any more on gnome 3.30… and especially not supported any more in future gnome versions .
Yes true. The Ubuntu developers decided not just discontinued Unity, but also they remove the patches to the core Gnome applications like is Nautilus and that was the way to support the Global Menu in that applications.
In that context a third party developer will have only a few alternatives as solution options:
1- Hack the GNOME core applications by himself and add again the menubar.
2- Continue the develop of an extension without give support to the GNOME core applications.
3- Discontinue the extension.
The option #1: This was frustrated when the Gtk developers also decided no support general-purpose modules
The option #2 This have not sense, as will be implement something that can not be used with the most commons applications.
The option #3 This was the option that was taken as is the more reasonable in this context.
Well yes, is worse than just the discontinuation of an extension, because was discontinued Unity, the intention of the global menu, the support fort it and then the last and less important thing, was discontinued that extension.
The discontinuation of the extension finally was as a normal consequence to all that previuslly things. Swimming against the river flow is of course difficult.