The original Softdisk games created by the id Software founders are now owned by Flat Rock Software. Flat Rock Software has decided to go ahead and open-source these old classics. Among the games with their sources out there are TheCatacomb, Catacomb3D, Catacomb, and Hovertank3D.
These classic id game titles are now GPL'ed and available freely via FlatRockSoft's GitHub and was confirmed by John Carmack. As with other id Software source releases, the engine is open-sourced under the GPLv2 but the game data still requires having a licensed copy of the content.
The original Catacomb was written in Borland Turbo Pascal 5.5 while at least Hovertank3D and the newer Catacombs are written in C++.
Here's a video for those unfamiliar with Hovertank 3D followed by the 1990 hit Catacomb: