There’s never been a CS2. Other than a version of the name of the set of Adobe programs (ie, Photoshop CS2)
CS 1.6 is the popular one. That version is about to turn 20 as well.
You’re probably thinking of Counter Strike: Source, the name they gave it when they released it built on the Source engine.
Then there was the current one, Global Offensive.
However, there’s a new one about to be released that I think is still being called CS2. Not sure if that’s the final name or not, I haven’t been following it very closely. But I think it’s due to release this month. Or sometime soon.