Historically, Arma 2 keys were sold purely for the DayZ mod. While the player base has largely moved to the standalone DayZ or Arma 3 , the modding scene for Arma 2 is still legendary.

Even if you purchase a "non-Steam" retail key, Bohemia Interactive allows you to activate it on the Steam platform for easier management and automatic updates. Activating a Product on Steam

If you are looking for an CD key, the most reliable way to obtain a valid, working code is through official digital storefronts or authorized retailers. Where to Get a Key