Cartoon Network UK's YouTube channel claims that Scooby-Doo is going to be in MultiVersus.
MultiVersus is full of different Warner Bros. characters, ranging from Adventure Time's Finn and Jake to DC's Superman and Wonder Woman. Scooby-Doo is also represented through Velma and Shaggy, but curiously the titular great dane isn't in the game at all so far. That's not what Cartoon Network UK is saying, however.
In the description for its repost of Multiversus' "You're with me" cinematic trailer, it says, "Join Jake the Dog, Finn the Human, Garnet, Bugs Bunny, Shaggy, Scooby-Doo, Superman, Batman and many others on the brand new game MultiVersus". Every other character on that list has been confirmed to be in the game so far, except for Scooby-Doo who hasn't yet been announced.
It's interesting to note that the description clearly doesn't mean the Scooby-Doo series as a whole, as it mentions Shaggy in the same sentence, implying that it's referring directly to the character instead. Although Scooby-Doo seems like a very obvious pick for the game and is referenced through player icons, he hasn't actually been a part of the various MultiVersus leaks over the past few months.
That being said, The Iron Giant only had one voice line in recent leaks, which led to many believing he was unlikely to appear in the game. We now know that isn't the case and the character is actually nearly finished, so it's possible that more is being done with characters behind the scenes than we know.
Tony Huynh, the game director for MultiVersus, was asked about Cartoon Network's description on Twitter. Sadly he said that it "looks like a mistake", which means that it's likely an error on Cartoon Network's part rather than a confirmation that the character is coming somewhere down the line. However, it's worth pointing out that Huynh didn't say that Scooby-Doo wasn't coming to the game, only that the YouTube description is likely a mistake.
It's worth mentioning that Tony Huynh previously reacted rather coyly to a fan saying that The Iron Giant would be a really cool pick for MultiVersus, so it's not like the director is going to give too much away on characters coming to the game.