Deathmatch Classic is a first-person shooter created and developed by Valve sometime after the release of Half-Life, and was released July 1st 2001. Deathmatch Classic is a homage to Quake Deathmatch and id Software, made with John Carmack's approval. It retains the same gameplay as Quake deathmatch. The models and textures have been revamped, however the player models are simply the ones from Half-Life. 7 maps were ported to DMC, 6 being original Quake maps.
During the development of Deathmatch Classic, members of the team creating the game attempted to port Zoid's Quake mod "ThreeWave CTF" to DMC. The project was never talked about publicly and it ceased development quickly. During the Half-Life 2 Leak in 2003, a full (but broken) build of ThreeWave CTF was found in the leak. The source code for ThreeWave CTF remains in the official Deathmatch Classic github repository.
"Many people at Valve cut their teeth on the original Quake deathmatch. It is still better than almost all of the new multiplayer games released.
Given our existing license agreement with id, we were able to create DMC for our own entertainment. It really reminded us of just how kick-ass
the game was and thought it would be a great reminder for everyone and a tribute to id's accomplishments if we were to finish it and release it.
So, we asked John Carmack about it, and he gave us the go ahead to do this. The key is that we have a license with id. Most of the world does not."
- Gabe Newell