I'm not aware of when it is incorporated into the difficulty naming wiki page, but since it is here I'm proposing to remove Outer Oni from difficulty naming for Expert+.
Going deep into the meaning for the current difficulty names for taiko, they originates from the game Taiko no Tatsujin, which includes Kantan, Futsuu, Muzukashii and Oni. There is also a hidden difficulty named Ura Oni, which is hidden under Oni difficulty. Japanese: 裏鬼 this has meaning of 1. hidden difficulty that needs to be explored under Oni; 2. A more condensed form under the original oni i.e. more condensed form of difficulty. Inner Oni is thus derived from Ura Oni from Japanese romanisation into English. Hell Oni has similar meaning into "Oni coming from hell" i.e. even harsher difficulty.
While the difficulty name "Outer Oni" doesn't really make sense. If it mean the opposite of Inner in this case, or 表鬼 in Japanese, "Surface of Oni" means a even lighter representation of Oni i.e. should be easier, (in-between Muzukashii and Oni)?
It doesn't really make sense to have Outer Oni being harder than Inner Oni.
I understand osu! taiko isn't in any way taiko no tatsujin, So the above elaborated why Inner Oni and Hell Oni can correlate in meaning, while Outer Oni does not.
Therefore, proposed change is to remove "Outer Oni" from the difficulty naming page.
Going deep into the meaning for the current difficulty names for taiko, they originates from the game Taiko no Tatsujin, which includes Kantan, Futsuu, Muzukashii and Oni. There is also a hidden difficulty named Ura Oni, which is hidden under Oni difficulty. Japanese: 裏鬼 this has meaning of 1. hidden difficulty that needs to be explored under Oni; 2. A more condensed form under the original oni i.e. more condensed form of difficulty. Inner Oni is thus derived from Ura Oni from Japanese romanisation into English. Hell Oni has similar meaning into "Oni coming from hell" i.e. even harsher difficulty.
While the difficulty name "Outer Oni" doesn't really make sense. If it mean the opposite of Inner in this case, or 表鬼 in Japanese, "Surface of Oni" means a even lighter representation of Oni i.e. should be easier, (in-between Muzukashii and Oni)?
It doesn't really make sense to have Outer Oni being harder than Inner Oni.
I understand osu! taiko isn't in any way taiko no tatsujin, So the above elaborated why Inner Oni and Hell Oni can correlate in meaning, while Outer Oni does not.
Therefore, proposed change is to remove "Outer Oni" from the difficulty naming page.