Did they want it to coincide with Valve’s 15th birthday or something? Eh, I don’t know. It must have been one hell of a typo, because the video only returned today, after a good 5 days.
The reason? Some sort of “typo” in the film. But that changed on the 19th of August, when they finally released Part Two… and it then changed right back again, as it was taken down a few hours afterwards.