It was one of the abilities removed during the leaked Beta press event videos along with
these others.
Unlike Overpower missing, which was confirmed to be a bug (there was a spot for it in video, just no icon/tooltip). The rest of these abilities are not present whatsoever in the spellbook.
Seeing as the other abilities on the list were confirmed by Celestalon as removed (Disarm, Spell Reflect, Shatt Throw, Hamstring & Skull Banner) it is HIGHLY likely this is not a mistake.
Likewise, they are removing Skull Banner, removing Demo Banner, I wouldn't be surprised at all if Mocking Banner gets nixed later, but that's just my assumption.
It also matches with them wanting to remove excess buttons and utility from classes. There is really no point to us having 2 defensive abilities, when most classes have one. Especially one that is as quirky as Demo Banner.