According to gnome-shell ticket 624: Remove support for app menus, the feature is going away.. See ticket:228#comment:14 : GNOME Plans to Move App Menus Back Inside App Windows.
A lot of churn and half-baked features. Moving further towards client-side decorations everywhere. Ouch.
Lots of ugly code gone!
More code removed: r22692.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/2163