Examples of this include; killing Wolverine in the comics, who at the time was the X-Men’s most marketable character; retconning Scarlet Witch and Quicksilver’s backstory as mutants; completely disbanding the Fantastic Four; prohibiting writers from creating new mutant characters so that Fox can’t use them; and the mutant race on the verge of extinction, while the Inhumans — who are essentially mutant stand-in’s in the MCU — suddenly found their numbers greatly increasing.
When Marvel plugs events called “Death of X,” you can see why this rumor gained some credence.