Because the game is old, the rendering pipeline is relatively simple, making it easy to intercept and modify.
CS 1.6 is still actively maintained and played on Steam. VAC is still active on official and protected community servers. Running an old OpenGL hack will result in a swift, irreversible VAC ban on your modern Steam account.
Eventually, VAC moved to and screenshot analysis (taking screenshots of the render output and looking for unrealistic colors like magenta). However, skilled cheat devs countered with:
This results in a "X-ray" effect: the walls appear solid, but the enemy silhouette bleeds through the geometry. This was the preferred method of "legit cheating" because it didn't look obvious on a spectator's screen.
