Board Thread:News and Announcements/@comment-61022-20151028130430/@comment-4651179-20151101195252

I'm going for the angle as how the events are perceived by us the readers rather than the characters in-universe.

Considering the Secret Wars with Deadpool is (supposedly) what really happened, the original Secret Wars would be rendered non-canon. Because reality wasn't altered so Deadpool hadn't appeared, they just don't remember him. It's not like One More Day in which MJ and Peter being married did happen, in this case Deadpool not being in Battleworld didn't happen at all.

(Also, it's implied it was Wasp who wished for Deadpool to be forgotten)