Is Counter-Strike: Source on Steam?

No. Counter-Strike: Source 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: Source on?

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

When did Counter-Strike: Source release?

Counter-Strike: Source was first released on October 7, 2004.

Counter-Strike: Source

Oct 7, 2004ShooterLinuxPC (Microsoft Windows)Mac

Counter-Strike: Source — frequently asked questions

Is Counter-Strike: Source on Steam?: No. Counter-Strike: Source 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: Source on?: Counter-Strike: Source is available on Linux, PC (Microsoft Windows), Mac.
When did Counter-Strike: Source release?: Counter-Strike: Source was first released on October 7, 2004.

About Counter-Strike: Source

Counter-Strike: Source blends Counter-Strike's award-winning teamplay action with the advanced technology of Source technology. Featuring state of the art graphics, all new sounds, and introducing physics, Counter-Strike: Source is a must-have for every action gamer.

Summary via IGDB.

Counter-Strike: Source on Wikipedia

Counter-Strike: Source is a tactical first-person shooter video game developed by Valve and Turtle Rock Studios. Released in October 2004 for Windows, it is a remake of Counter-Strike (2000) using the Source game engine. As in the original, Counter-Strike: Source pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won either by completing an objective or by eliminating all members of the enemy team. The game was initially bundled with all retail and digital copies of Half-Life 2, before being released standalone.

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