It’s always been shocking to me how little content there is in total around Super Sonic. There are only three major pieces of concept art for the form that have ever been officially released:
This sketch page of the form and a bunch of boss designs from Sonic 2.
This alleged redesign of the form from Sonic Adventure’s development, courtesy of Naoto Oshima of all people.
And this famous wallpaper-like design from the development of Sonic Frontiers.
All of this is super weird. Oshima designed the Sonic 2 version of Super Sonic, but that’s all we know. We have no existing concept art for Sonic 3, despite how integral the form was in that game’s story. Sonic Frontiers is the most front and center Super Sonic has ever been, yet we only have a single piece of concept art that shows him. And Sonic Adventure’s strange redesign has no explanation, despite it clearly showing them trying to get away from the super saiyan inspired version of the form.
I think SEGA at large has always been on edge about Super Sonic, as it is a sort of grey area with him. I don’t think it’s copyright infringement, but I think SEGA as a whole doesn’t want to take the risk. He’s used in a lot of games, but he has relatively little merch made until recently, and SEGA for a long time seemed to be wary of putting him in any other media.
Plus, let’s not forget the Nazo concept, which was a scrapped Super Sonic redesign for Sonic X that became a famous fan villain.