Non-MCU films featuring Marvel characters, including the X-Men series and the Fantastic Four movies, get their own Marvel Legacy category on Disney+. The entertainment giant's streaming service currently features all of the MCU movies and has even organized them according to different watch orders, allowing fans to see them according to the universe timeline, by phase, or pick and choose. But Disney+ also includes a number of other Marvel movies that aren't part of the MCU, having been made by 20th Century Fox as part of a separate rights agreement.
That studio was famously acquired by Disney in 2019, gaining the rights to all of the studio's movie and TV properties, bringing shows like The Simpsons under Disney's control. As a result, Disney now owns the non-MCU Marvel franchises too. That's why movies from the X-Men franchise and the Fantastic Four movies, which all feature Marvel characters, are available to watch on Disney+. Previously they were listed alongside the MCU movies in the Marvel category of the site, with only their absence from the Marvel Cinematic Universe category marking them out as different. But that's just changed.
Along with the new MCU categories, Disney+ has placed the non-MCU movies into a category called Marvel Legacy Movies. In the USA, the category currently contains two Fantastic Four movies as well as five movies from the X-Men franchise, and should get more added once the streaming rights of various franchise movies on other platforms revert to Disney. You can see a screenshot of the category below:
The move has been made not only because it makes it clearer to casual audiences which movies are part of the MCU and which aren't, but also to prepare for the characters from those franchises to make their debuts in the MCU. It's long been rumored that the Fantastic Four and X-Men characters will be introduced to the MCU soon, most likely in phase 4, resulting in iconic roles like Professor X and Wolverine needing to be recast. As a result, casual fans will be even more confused by the different versions of the characters, particularly if they want to watch them on Disney+.
By separating them into different categories, Disney is simply making it easier to find different versions of each franchise. The move is also potentially a sign that Disney is gearing up to announce rebooted versions of each franchise, which should get fans excited. Anticipation for seeing the likes of the X-Men alongside Spider-Man and the rest of the Avengers is high, though of course, Disney may choose to produce solo movies first before introducing the characters in the wider MCU.
Source: Disney+