Dust2 was added in CS 1.1, March 2001
David Johnston's Dust2 entered the map pool on March 13, 2001 — barely a year after CS's first standalone release. It's the most-played FPS map in history and has been remade three times (CS:Source 2005, CS:GO 2017, CS2 2023).
Source: Counter-Strike 1.1 patch notes