UPDATE: As the original post is from a graphic designer, it now seems likely this is indeed a fake leak. We've reached out to Nintendo for comment, though we can say with confidence that Sora and the gang will not be coming to Smash later this month.
Super Smash Bros. Ultimate has easily surpassed its predecessors in seeing the most crossovers from other franchises in the gaming industry. The game features many non-Nintendo gaming icons, from its inclusion of fighters like Simon Belmont from Castlevania and Joker from Persona 5 to stickers like those of Chris Redfield and Jill Valentine from the Resident Evil series, and it's about to get more with new Spirits featuring characters from Kingdom Hearts if a new rumor holds true.
According to MatttGFX on Twitter, the Twitter account Nintendo Versus posted a Kingdom Hearts-related tweet to their feed, indicating that, for a limited time, Spirits of characters from Kingdom Hearts would be coming to Super Smash Bros. Ultimate in the middle of December.
However, Nintendo Versus had deleted the tweet for unknown reasons, but one potential reason likely involves the fact that it was an unwarranted leak. The tweet can be seen below.
NintendoVS just posted and deleted this pic.twitter.com/XhkE6GTVrL— Mattt 🦐 (@MatttGFX) November 28, 2019
The tweet does not provide further details. While nothing has been confirmed yet, as illustrated by the tweet's removal, the addition of the Kingdom Hearts Spirits to Super Smash Bros. Ultimate would be made on December 13th. The characters include Sora, Riku, and Kairi and will, if the information's true, appear for 5 days on the Spirit Board.
As to why Nintendo Versus deleted the tweet, there are many potential explanations. If it's true, then it may be as simple as a mis-timed release. Regardless, seeing the Kingdom Hearts trio in Smash would certainly be a treat.
For those unaware, Spirits' primary use in Super Smash Bros. Ultimate is in the World of Light mode. There are a variety of Spirits, including offense-based and defense-based. Mainly, Spirits are used to boost various stats in World of Light, acting as power-ups.
This does bring up the topic of what other franchises could come to Super Smash Bros. Ultimate. Already, franchises like Resident Evil have appeared, as well as plenty of others. Only the future will tell what other franchises may come.