Spoilers ahead for Spider-Man: Far From Home.
Mysterio's explanation of a Marvel Cinematic Universe Multiverse might have been shot down as a deceptive ploy in Spider-Man: Far From Home, but Marvel Studios' President Kevin Feige is re-confirming that the MCU multiverse is indeed real. When asked if fans will never see a MCU multiverse, Feige shot it down pretty quickly.
"No, it just means [Mysterio] was full of s**t," Feige told Fandango. "I mean, in Doctor Strange, we hear the Ancient One talk about the multiverse, so we’ve already established it as a thing."
Back in 2016's Doctor Strange, the Ancient One mentions that the Marvel Cinematic Universe is "only one of an infinite number," telling Strange he's part of a "Multiverse" - but stops there. However in this video from February 2017 promoting the film's release on home video, Strange actor Benedict Cumberbatch lays it out in even more detail.