My guess is they'll hint at a shared universe at the point they feel like taking advantage of that shared universe. Until then, talking about mutants creates more of a mess than not talking about them. The Fantastic Four get powers - mutants! get them. Oh, they're not mutants. That means it's possible to get powers without having genetic differences from birth. Maybe mutants aren't the next evolution of humanity. If we're OK with the Fantastic Four as heroes, why not the X-Men?
Well, human/mutant tension and oppression has defined all three (plus The One We Don't Speak About) main-team
X-Men films, and, given that
DOFP ends on a hopeful note in the 70s, maybe there's a willingness on the studio's part to start moving past that particular dynamic? As in, sure, there'll always be those wary of mutants, but for the most part, so long as things don't get too crazy, people will be content to live and let live. Also, all the
X-movies so far have been the team containing threats and attacks from Magneto; maybe the 1980s events of
Apocalypse will show that the Xavier Bunch is useful to have around in case of particular emergencies.
Furthermore, the FF4 are just four people, made special by a one-of-a-kind accident, so the world may not fear them as much as they do mutants. Kinda like how we still have a very vicious war on drugs, but certain overt and unabashed celebrity potheads like Willie Nelson have been tolerated and even celebrated by people of all stripes for decades. And, unlike many/most mutants living in secret, they're publicly identifiable and all that.