During a recent interview, Kevin Feige was asked point-blank about the alleged R-rating for the Black Widow movie (which hasn’t even technically been formally announced) and he was very quick to shoot it down.
Black Widow does not yet have a release date, but we would expect to see it debut sometime in 2020.
But Black Widow is already firmly established in the PG-13 universe that is the MCU.
There have been rumors floating around that the upcoming Black Widow solo movie, which will finally see Scarlett Johansson get to go on a solo adventure in the Marvel Cinematic Universe, was going to be R-rated.