Although it really shouldn’t have happened in the first place, you won’t have to worry about paying for Super Sonic in Sonic Forces anymore. According to the official Sonic PR, the DLC will remain free forever:
More good news: we’ve updated the plan for Super Sonic DLC in Sonic Forces!
Starting today, the Super Sonic DLC will be free – forever.
— Sonic the Hedgehog (@sonic_hedgehog) January 24, 2018