The RetroArch devs promised that more emulator cores would be coming to Steam soon, and the first step in that process has happened. Whether it was called the Genesis or Mega Drive in your youth, Sega's 16-bit console has gotten extra representation in the Steam version of RetroArch with Picodrive.
Two other RetroArch emulators available on Steam Genesis and Mega Drive games – Genesis Plus GX and multi-system emulator Final Burn Neo – but Picodrive goes a step further with for 32X games. That's something even the official Sega collection on Steam doesn't offer.
The RetroArch folks note that this is just the first additional emulator Valve's service, as "new cores are on the verge of being approved on Steam". As they've detailed in the past, Valve restrictions prevent RetroArch on Steam from using the same in-app emulator s and updates that the standalone version provides, and thus the developers have to get each new emulator approved as free DLC listed on the store.
For old games that are still worth playing, you can follow that link.