I thought it was, but only with significant margin: https://www.marketwatch.com/story/interactive-brokers-says-its-will-now-accept-bets-that-bitcoin-will-tumble-2017-12-13

If you have different info I'd love to see it.

Either way, as @haejin pointed out, it's somewhat irrelevant because these things often become post hoc justifications for price.

I think it's worth understanding the implication of shorting in the market though, since in the past, it was a very one sided market.