According to Aaron Webber from Sega, Super Sonic was originally never meant to be in Sonic Forces at all, which was why the transformation was relegated to DLC. While the DLC was supposed to be free for a limited time, it has since been made free. [Thanks, NintendoEverything!]
According to Webber, Super Sonic was never meant to be playable in the original game, as the game does not make use of the Chaos Emeralds in the story at all. It was decided mid-development that DLC would be made for it, which was where the pricing decision was made. Webber said that the team had worked hard on the game, and that it was pretty fair for there to be an uproar despite being a misunderstanding.
Previously, the files for Super Sonic were found in the Day 1 patch of the game.
Sonic Forces is currently available on PlayStation 4, Nintendo Switch, Xbox One and PC.