Is Counter-Strike: Condition Zero on Steam?

No. Counter-Strike: Condition Zero is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.

What platforms is Counter-Strike: Condition Zero on?

Counter-Strike: Condition Zero is available on Xbox, Linux, PC (Microsoft Windows), Mac.

When did Counter-Strike: Condition Zero release?

Counter-Strike: Condition Zero was first released on March 23, 2004.

Counter-Strike: Condition Zero

Mar 23, 2004ShooterXboxLinuxPC (Microsoft Windows)Mac

Counter-Strike: Condition Zero — frequently asked questions

Is Counter-Strike: Condition Zero on Steam?: No. Counter-Strike: Condition Zero is not distributed on Steam, so there is no Steam Charts or SteamDB player count for it. Its live popularity is tracked here via Twitch viewership instead.
What platforms is Counter-Strike: Condition Zero on?: Counter-Strike: Condition Zero is available on Xbox, Linux, PC (Microsoft Windows), Mac.
When did Counter-Strike: Condition Zero release?: Counter-Strike: Condition Zero was first released on March 23, 2004.

About Counter-Strike: Condition Zero

With its extensive Tour of Duty campaign, a near-limitless number of skirmish modes, updates and new content for Counter-Strike's award-winning multiplayer game play, plus over 12 bonus single player missions, Counter-Strike: Condition Zero is a tremendous offering of single and multiplayer content.

Summary via IGDB.

Counter-Strike: Condition Zero on Wikipedia

Counter-Strike: Condition Zero is a first-person shooter video game developed by Ritual Entertainment, Turtle Rock Studios, and Valve, and published by Sierra Entertainment and Valve. The follow-up to Counter-Strike (2000), it was released in March 2004 for Windows. Condition Zero utilizes the GoldSrc engine and has a multiplayer mode, which features updated character models, textures, maps and other graphical tweaks. It also includes two single-player campaigns; Tour of Duty and Condition Zero: Deleted Scenes.

Source: Wikipedia (text under CC BY-SA 4.0).