He is talking about this abomination: https://www.wowhead.com/news/blizzard-blog-play-a-death-knight-in-wrath-of-the-lich-king-classic-and-get-a-328758, Play a Death Knight in Wrath of the Lich King Classic™ Before 28/11 and Get a Mount in WoW®.
I don’t know when it will end, but just do it now. Takes about three hours and you are done. Just don’t come to Hydraxian Waterlords. Enough DKs solely there for the mount there already.^^