This generator has been in beta up to June 2016.
Due to lack of announcements related to specific plans for mobile phones from Microsoft, at least during 2016, market trends, among other reasons, GeneXus has stopped the development work for this generator.
Nevertheless, the bits of the generator are still included in GeneXus X Evolution 3 Upgrades and code generation for this platform keeps "as is". The generator is not included in GeneXus 15.
You can watch Microsoft’s Mangarelli -Director of Technology, Developers & Platform Group, Microsoft Latam- announcing the new GeneXus breakthrough at our 22nd GeneXus Event (October 1st, 2012).
Or watch Alejandro Silva's - Windows 8 Generator Development Lead at Artech - presentation at the same meeting. This is a more technical overview of this generator. Slides can be seen and downloaded from here