Why did this work? Because the Deviance client had stripped out the call to the authentication master server. The server you were joining only checked to see if a key existed in the registry slot, not if it was valid. Consequently, thousands of players who never paid for the game used these "Deviance keys" to play online.
In 2014, GameSpy shut down its master servers entirely. Suddenly, every copy of Call of Duty 2 —legit or pirated—could no longer see the server list. The "Deviance" fix became the only fix. The community rallied, creating workarounds like the "CoD2 Revive Launcher" and updating the Deviance project to point to community master servers. Call Of Duty 2 Deviance Cd Key
Here is the truth:
Today, if you dig up your old CoD2 disc—or buy it on Steam—do not waste time searching Google for a dead key. Instead, look for the "CoD2 Community Client." The Deviance project may be dead, but its spirit lives on in the private servers that still run today. Why did this work
Introduction: The Golden Age of Modding