I think BTC-China always had fees until recently perhaps.  What happened in Dec/19 is that they lost the deposit channel via bank transfer. They came up with a voucher system that apparently was not popular, while Huobi and OKCoin found a way to continue using banks for deposit.  I don't know exactly what is the reason for their recent revival: they accept USD and HKD deposits, they have a mobile trading app, they may have eliminated the trading fees...