Blizzard is taking a page from Disney with this one: in "approximately one week" the Digital Deluxe edition of Mists of Pandaria will be sent to the Blizzard Archive. Meaning: Unable to be acquired for an undetermined amount of time. If you don't have the Imperial Quilen mount or the Lucky Quilen Cub pet but were considering picking them up, upgrade your account before it's too late. Much like your favorite Disney film, the Mists Digital Deluxe might return one day, but who knows when.
Personally, I'm curious why the Digital Deluxe is going away at all. Digital items have no real scarcity beyond the artificial. It's possible we'll see Mists of Pandaria become a part of a Warlords of Draenor battle chest, rendering a digital deluxe of anything prior to Warlords a complication. When the Digital Deluxe returns, we might see it in the form of buying the included digital items individually rather than as a package. That's only speculation, however. You can view the full blue post behind the cut below or on Battle.net.