Official or semi-official? Never, unless github's server goes FOSS at some point. When someone attempted to get github mirrors of GNOME or GCC declared "official", the official response was that anything hosted on a proprietary git hosting site should never be official.
Interesting; thanks for the correction. Last time I'd seen that discussion, I thought the conclusion was that it was a supported mirror (as in, the GNOME project would keep it up to date), just not an endorsed mirror (as in, the GNOME project doesn't officially recommend its use).