Is not the new content which Valve has released. Instead it's a multiplayer mod from the original Half-Life which was all about bringing the fabulous-ness of capture the flag to Valve's massively successful shooter.
Here's some history and a glimpse at the newly discovered code. It's a pretty fascinating story.
This tale has been two decades in the telling, starting with a mod for the original Quake back in 1996 and a modder who worked for id and later Valve. A huge amount of work has been put into making it possible to play these maps on the old Half-Life engine, which is a pretty amazing thing to be able to do for something released back 1998.
If you're curious, you can head over here and download it.