I'm sure it's not the best way to do things, but the campground is a total wasteland when everyone leaves, there's just piles of shit everywhere. They have a lot of workers come through to clean stuff up, but I bet they'd prefer if you didn't leave things behind if possible.
2 years ago my car was one of the very last out of the campground on tuesday and it was a fucking mess. Hundreds of tents with sleeping bags left behind, coolers, food of all kinds, tarps, grills, anything people can bring there was left behind. That sight was really depressing and I highly encourage everyone to pack out what you bring with you. You do it when you go camping normally (or you ought to), why not do it at Sasquatch too?