Earlier quoted context omitted.
This project has been going on for over 25 years. It used a Sonic Doom wad I made in the 90s as a starting point. If Sega really objected to it, they would have dropped the hammer by now.
Wow! How did your WAD start? What were your goals? What do you think of how others have done?
One of the key factors in the history of Sonic that I didn't know until recently was that he was a total flop in Japan. In fact the Mega Drive struggled to gain traction there. That's why they kept trying to reinvent the formula into the 2000s and beyond; from Sega (of Japan)'s perspective, unlike Mario, they hadn't established a gameplay template that they knew would be a worldwide hit, only a character they knew was popular in the USA and Europe.
But the SRB developers got it. They knew what made classic Sonic work, and sought to replicate that with the strengths—and limitations—of the Doom engine and produced an absolute banger of a project.