Fan-made projects based on existing video game franchises commonly stand on shaky ground in the modern era, with many companies obligated to shut down fan games to protect their IP.
Sega is usually more relaxed in this regard, and Katie Chrzanowski, Associate Influencer Manager at Sega of America, recently reached out to Sonic fans to address the matter.
Source: N4G PC SEGA has no issue with Sonic fan games so long as no profit is involved