It's common American slang. She's still down for vandalizing his car.
She's down for it = she's ready, she's available, she's looking forward to it, she's psyched, she's hot to trot.
I think it evolved from writing things down on lists. For instance:
- You: How many tomatoes will be in my vegetable order?
- Me: I have you down [on my list] for a dozen.
Over time, the meaning of down for has expanded. So now, if we want to do something, we often say we're down for it.
"I'd still be down" is short for "I'd still be down for it," easily understood by a native speaker.
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