@Stan` and I have actually started a repo for Empires Besieged here: https://gitea.wildfiregames.com/0ad/empires_besieged.
The last change was 11 months ago, but I'm working on things in DE (Guptas and Sasanians; a big thank you to @Duileoga and @Lopess) which will directly translate to EB at some point in the future.
So, whether EB becomes it's own "game" or an "era" or an "expansion" or a "release" is up for debate. I personally advocate for a "0 A.D. Eras" concept, where players can swap between eras (Empires Ascendant, Empires Besieged, Masters of Bronze, Millennium AD) and only play those civs, or choose to play with all civs available at once.