The infamous feud between 21st Century Fox and Marvel Studios resulted in the latter company acting in a hilariously petty fashion by erasing the X-Men from as much media as possible - even retroactively. The Marvel Cinematic Universe was unable to include key players until 2019, when a landmark deal was struck between Disney and Fox that resulted in the acquisition of the Fantastic Four, Galactus, the Silver Surfer, and the X-Men. But in 2016, the feud was still going strong - resulting in Marvel editing the X-Men out of a classic cover.
While it's well-known that a well-received superhero film boosts comic book sales, the reverse is also true; a film promoted extensively in the comics translates to positive box office results within fan circles - the type of audience who would see a film more than once. But Marvel didn't wish to see their direct competitor in the superhero game - 21st Century Fox - benefit from their comics. Thus, the X-Men all but disappeared from most event comics (unless they were fighting another team - see Avengers vs. X-Men and Inhumans vs. X-Men).
In 2016, Marvel revealed their annual desk calendar of superheroes. The calendar depicts a look back at various classic covers from the Golden, Silver, and Bronze Ages of comics. A fan favorite cover, Sensational She-Hulk #1, is featured - but contains a notable edit. In the original cover, She-Hulk breaks the fourth wall (as is her custom) and addresses the reader: "If you don't buy my book this time, I'm gonna come to your house and rip up all your X-Men." But in the edited cover, she says "If you don't buy my book this time, I'm gonna come to your house and rip up all of yours."
The She-Hulk edit is an astounding petty and juvenile move from a multi-million dollar publisher. Not including the X-Men in modern stories moving forward after 2010 is one thing, but retroactively erasing a classic cover (not to mention a good punchline, since She-Hulk sales weren't nearly approaching the numbers of the X-Men at the time) is, to put it mildly, a slap in the face to both She-Hulk and X-Men fans. It's worth mentioning that the rest of the calendar does not include a single X-Men cover (0r Fantastic Four cover for that matter, and the omission of Marvel's First Family is perhaps just as glaring).
As of 2019, the X-Men are back in the hands of Marvel - and coincidently, the company has included them in events once again, even promoting the new X Lives of Wolverine/X Deaths of Wolverine series for January 2022. But Marvel's repeated attempts to erase their own history out of spite for 21st Century Fox remains a black mark on the company. Hopefully, now that Disney, and by extension Marvel Studios, has acquired the X-Men rights once again, this should mark the end of Marvel's attempt to remove mutants from the comic continuity.