I agree with this the Ultimate X-men stuff put a pretty good guideline for mutants in modern era ,Only handful of Mutants like Apocalypse, Wolverine, Selene, Namor, Magneto, Professor X,Mystique and couple other need deeper explaining. Otherwise it is just explaining why mutant population suddenly is expanding/exploding over 20 to 30 year period which isn't real that hard. Almost no old history of the X-men is "needed" in the late 80s and 90s mutant population exploded(but still low enough to not be noticed as big thing plus other superhuman are going in world as well) and today this year the X-men are coming out of shadows. It is simple as that.
Before 70's only a couple in world
70's like 10 or 20
80's like 50 or 60(Magneto and Professor X discover they are mutants)
90's like 100
Early 2000's like 500 (mutant phenomenon announced to world)
today 1,000 and growing(X-men and Magneto revealed themselves to the world)
Every X-men story that is need to be told will happen from now on. Like the Ultimate X-men you are adapting old X-men stories to fit the modern era. Changes that have to made Magneto isn't Holocaust survivor but his parents were he gets tattoo like their own as symbol never to forget what happened and I can't think of anything else really. Why didn't we see mutants in the MCU before? They weren't really that much then. Why are mutants something to fear? Because every generation of mutants seem to get stronger than the next. The longer mutants are around more likely one turns into a ticking time bomb and destroy the world. That is what makes them different from Spiderman or Captain America. I think that is how I would do it.