If not a complete ignoring of The Last Stand, at most a mere passing mentioning
I think it's too late for that.
Donald Draper wrote:
Remember when First Class came out the question everyone had was if it was going to reboot the whole series.
As I recall, the appearances of Jackman and Romijn were taken as evidence to the contrary, and a lot of people argued that the continuity problems could be explained or rationalized in various ways. But as we've seen in Star Trek, a "reboot" through time travel isn't the same thing as a Batman Begins
-style reboot in which you completely start from scratch with an entirely different cast and the preceding films are assumed to have never happened at all.