Bittrex is a very careful cryptocurrency exchange and they prefer quality over quantity (of course this can be debatable). As for Bitcoin Gold or BTG there can be strong reason or reasons why Bittrex decided to delist the coin. Anyway, in my perspective this coin is of no use and it came alive just for the people behind it to make money for themselves and now that the goal had been meet then it is time to let the coin die its natural death. Who needs BTG, by the way? With less demand, it is time to let it go!