Generally speaking, the various Wikipedia projects tend to have an inherent bias against free, open-source software. Their sourcing policies are stuck in the pre-Internet era, focusing heavily on professionally-edited publications that are rare in the FOSS niche (particularly when it comes to games). The German Wikipedia is reputed to be the worst of the lot when it comes to notability, so it's unsurprising that 0 A.D.'s article was deleted. As far as I know, the project was taken over by deletionists a few years ago and they've been on a heavy-handed rampage ever since (note the huge swath of Linux distributions also up for deletion on that same archive page).