Deathmatch Classic was released as a free mod for Half-Life on on June 7, 2001, and was subsequently included in an update for Half-Life the following month. When Valve ported its GoldSrc titles to Steam, the mod was changed to a paid game, along with adding a new map and main menu background. Owners of retail copies of Half-Life that register their Half-Life CD key via Steam have been grandfathered in, receiving a Steam copy of Deathmatch Classic for free.
The game only contains its titular free-for-all deathmatch mode. A port of David "Zoid" Kirsch's popular capture-the-flag mod for Quake, "ThreeWave CTF" (or simply "3wave"), was planned, but ultimately abandoned, despite having new assets and maps created for it.
The source code for Deathmatch Classic and the unfinished ThreeWave CTF expansion can be found in the Half-Life SDK code repository, in the
dmc subdirectory. A fork updated to properly compile with Visual Studio 2019 Community Edition can be found on GitHub.