It's "open enough", meaning that you can just walk around the gates and go ride. I think that they use it for DC United games and concerts, so while it gets cleaned up, there might be broken bottle glass here and there, too.
I went there a few days ago, and they had put barriers curving from underneath the Metro track going across the exit from Turn 3, possibly to prevent people at the baseball field/training pitch from driving onto the rest of the lot. The barriers were spaced apart with gaps of about one to two yards, though, so they didn't block bike access entirely; you'd just have to be careful with your line.
I saw a couple security vehicles driving around, too, but neither of them stopped to talk to me or ask me anything. I figure that they're cool with cyclists -- although I've never seen other riders besides me and my coworker.
As far as why they haven't kept racing there, I don't know for sure. I think it was a conflict between the race organizers, sponsors, and/or IMSA (assuming that it was an ALMS event).