Warning: The following contains spoilers for Venom: Let There Be Carnage.
After years of anticipation for the Marvel Cinematic Universe to finally reference mutants, Venom: Let There Be Carnage did it first. Thanks to the complicated rights situation with various Marvel properties, Disney's MCU movies weren't allowed to reference mutants, or even include variations of the word "mutant" until after they purchased the X-Men rights through their acquisition of 20th Century Fox, but now that they finally have the chance to do it, Venom: Let There Be Carnage beat them to the punch.
The lines have always been a little blurry, particularly with Avengers: Age of Ultron introducing Scarlet Witch and Quicksilver, who are mutants in Marvel comics, but it turned out the MCU couldn't actually refer to them as mutants, or use the word "mutant," since they only had rights to them via their status shared status as Avengers. Kevin Feige confirmed mutants are on their way to the MCU, though there's no movies on the current slate that are expected to include them, at least in any significant capacity.
In light of the MCU's mutant restrictions, it made it jump out even more when Venom: Let There Be Carnage introduced Shriek, who is explicitly a mutant in Marvel comics lore, although her relationship to Spider-Man characters mean she falls under Sony's collection of Marvel rights; however, unlike Scarlet Witch and Quicksilver in the MCU, Venom: Let There Be Carnage didn't beat around the bush when it came to where Shriek's powers come from, confirming immediately they were the product of "mutations," dropping the M-word the MCU was historically restricted from using.
It's not clear if Sony's particular rights ownership has always given them the ability to reference mutants in this way, or if the movie's MCU connection meant Disney granted them permission, despite not having introduced mutants, or even used the word yet themselves. Granted, referring to Shriek as a mutant isn't nearly as loaded of a phrase as it would be to refer to Scarlet Witch as a mutant. Shriek is far more connected to Carnage and other characters in Spider-Man's wheelhouse, whereas calling Scarlet Witch a mutant immediately conjures thoughts of the X-Men.
Either way, with Venom joining the MCU at the end of Venom: Let There Be Carnage, it makes Shriek the closest the MCU has ever actually come to official mutants, even if she didn't cross dimensions with Venom during the movie's credits. The MCU's first official reference to mutants will surely be a momentous occasion, but it won't change the fact that Venom: Let There Be Carnage did it first.
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