Masahiro Sakurai might be keeping Mai Shiranui out of Super Smash Bros. Ultimate in order to preserve the age rating, but the fans have decided to put her into the game themselves through the use of Mii costumes and the Stage Builder mode.
A special Super Smash Bros. Ultimate Direct was held for Terry Bogard when he was added to the game as a DLC character. Terry comes with a stage called King of Fighters Stadium, which has background cameos from different SNK characters. Sakurai pointed out that Mai Shiranui from the Fatal Fury series would not be appearing as a background character. In the Japanese version of the Direct, Sakurai mentioned that including Mai could have increased the CERO age rating for the game, while the English dub of the Direct added a joke about how Super Smash Bros. Ultimate is for good boys and girls of all ages.
Mai might not be appearing in Super Smash Bros. Ultimate, but the fans have used the more creative modes of the game to include her. According to Automaton Media, there are numerous Mii costumes based on Mai that can be downloaded online, while there are numerous levels in the Stage Builder mode that are based on Mai.
A common theme of the Mai stages is focusing on the more... sexualized aspects of her design, with stages that use her breasts as squishy platforms for the fighters to jump on.
The Stage Builder mode is custom made for controversy and it took all of five minutes for fans to flood the servers with stages in the shape of the male reproductive organ at launch. Once the initial wave of rude stages died down, the mode became a vehicle for more creative endeavors, as well as an outlet for making levels based on the latest memes. Sakurai might not have wanted to include Mai due to her design, but the fans have spoken and there is now more Mai in Super Smash Bros. Ultimate than ever.
Source: Automaton Media