I've listed in chronological order key pieces of information that I find to be of utmost interest in regards to Geno.
May 13, 2018 - Day of the Mysterious
tweet.
November 1, 2018 - During the Super Smash Bros. Ultimate Direct, a player icon with an image of Geno is seen within the preview of the
Battle Arena mode.
December 7, 2018 - Super Smash Bros. Ultimate is released worldwide, and from there players who played World of Light eventually came upon the Legendary Rank Attack Type Primary Spirit, Geno (using the same image as the player icon shown in the November 1st Smash-focused Direct). In addition, players who acquired all Mii Costumes would come to notice that the Geno Mii costume was missing.
July 12, 2019 - A fellow
Geno Bro reports on two Super Mario RPG tracks being struck down by Nintendo at around
May 15, 2019. A day later, PapaGenos uploads a video reporting on this finding.
July 30, 2019 - Nintendo uploads the Hero presentation
video made by Masahiro Sakurai. Upon the reveal of the Mii costumes being released alongside Hero, Geno's Mii costume is still missing, as well as the Chocobo Hat. Mii costumes only centered within the Dragon Quest franchise.
September 4, 2019 - During the Nintendo Direct
presentation, when the Super Nintendo NSO Library was announced, among the 20 titles, Super Mario RPG: Legend of the Seven Stars was missing, despite being one of the SNES most memorable titles.
October 19, 2019 - PapaGenos uploads a
video discussing a "leak" that claimed Crash Bandicoot would be the 5th DLC Fighter, and within said video, a list of Mii costumes was shown. The list contained Mii Costumes that were in fact released alongside Banjo & Kazooie, with the exception of three, these being a supposed Mallow, and/or Smithy costume, and the Chocobo Hat. The list is dated July 19, 2019.