Controversies of characters getting deconfirmed in "Super Smash Bros. Ultimate"
In March 2018 at a Nintendo Direct, a new unnamed Super Smash Bros. game was announced for the Nintendo Switch, and then three months later at E3, it was officially renamed Super Smash Bros. Ultimate followed by a release in Decemher 7th, 2018 to universal acclaim and high sales.
However, ever since the second announcement at E3, the game has been subject to a great many controversies before it even came out, most notably the one about highly-requested characters being deconfirmed to appear in it in any shape or form.
After the game's second announcement at E3 2018, Nintendo had been giving out more information about it in the next few weeks regarding characters, content, and Assist Trophies, including Waluigi, which basically confirmed that he will not be a playable character and will instead be an Assist Trophy just like the past two installments, which led to a lot of controversy from the Smash community, especially the character's fanbase which started bullying and harrassing Sakurai on Twitter, despite all the hard work and effort he'd been putting into the game.  In the same annoucement, characters like Isaac from Golden Sun, Shadow from the Sonic series, and Ashley from WarioWare where also confirmed to return as Assists, angering the fanbase so much that one fan actually commited suicide after finding out about the latter's deconfirmation as a playable fighter.
In early November 2019, a presentation of Terry Bogard from SNK's Fatal Fury, who was revealed to be the next DLC character after Banjo and Kazooie two months prior, took place explaining Terry's moveset, stage, alternate colors, and victory poses, and at one point during the presentation, Sakurai stated that Mai Shiranui, another character from Fatal Fury as well as one of the most iconic characters of SNK and gaming as a whole, would not appear in the game, not even as a cameo, stating that "Smash is a game for good boys and girls of all ages", leading to a lot of internet memes about this statement as well as some criticism about Mai's absense from the game, as Bayonetta (who comes from a M-rated series) was allowed to appear in the game and toned down like in her previous appearance in Smash for Wii U and 3DS. The reason for this deconfirmation is that Sakurai had originally intended for the character to actually appear in it as a background character in The King of Fighters Stadium (Terry's stage), albeit toned down, but CERO, an age rating system in Japan, would not allow that due to the game's ESRB rating, which is E10+, and both Sakurai and SNK had decided not to include her at all. 
One year later in mid December 2020, another presentation was held by Sakurai, and this time, it was Sephiroth from Square Enix's Final Fantasy VII, who was announced weeks ago at the Game Awards, and during the presentation, a new Mii costume was confirmed to be in the game, which was based of Geno from Super Mario RPG, which caused a lot of backlash from the community due to the fact that Geno was one of the most requested characters of all time to appear in a Smash game ever since Brawl, and it led to a lot of people thinking that he would never be playable because of it, similar to how King K. Rool was announced as a Mii costume in the last game, which caused a similar controversy.
On June 2021, during the presentation of Kazuya Mishima from Tekken, who was announced as the next DLC fighter after being revealed for the game at E3 in the same month, and like Sephiroth's presentation, several more Mii costumes were confirmed, and like Geno, most of them were based off of highly-requested characters to be playable in Smash, and those were Dante from Devil May Cry and Shantae from the franchise of the same name, both of which, again, sparked some huge backlash from the Smash fanbase because of how much they wanted these characters to be playable in the series, and thinking that these characters just being Mii costumes decreased any chances of that ever happening.