Prof9 first noticed the vanishing act around October 10th. The helmet - and only the helmet - was still listed then.
Developed by Flashman Studios, the gear first launched on Xbox Live in 2012 to celebrate the franchise's 25th anniversary. It consisted the Mega Buster, Mega Helmet, and even costumes of the original six Robots Masters. At the time of this writing there's been no word from Capcom, Microsoft or Flashman Studios. If we learn anything more, we'll let you know.