Sonic The Hedgehog CD (Sega Master System) is a ROM Hack. See ROM Hacks
Sonic The Hedgehog CD (Sega Master System) (or just Sonic CD (SMS)). is a demake of Sega's Sonic CD, a game originally made for the Sega/Mega CD. The hack is currently being developed by Chancanger.
On the 5th of January, 2018, a YouTube video was posted on the developer's YouTube channel, showing his progress. This showed some messed up levels that were in the process of begin put together.
No update videos have been released of over a year though, so it's unknown if it is still in development.