This was done a day or two before the core Zpacks gang left for a trip to Patagonia, and not wanting to be a douch and email them while they were out of country, I figured I would wait until they got back home.
I have just finished corresponding with them and they have confirmed that the Zero and XS Zero backpacks 'are no longer being produced'.
They did confirm with me that they are 'working on a new, small volume, minimalist' backpack that is intended to replace the Zero backpacks, but there are no details on this new backpack at this point in time.
My guess (and that is all it is) would be that it would be somewhere in the 30-45 pack volume size, basically filling the current Zero pack size, but if it will be a stripped down version that people can add features onto, is totally unknown.
I will, of course, update with further news as I get it and/or it is released by Zpacks.