If you look at seat61.com about Beijing - Xian, deluxe soft sleeper is still offered on that Z train. I guess the regular soft sleeper between Beijing and Shanghai is just so much more popular than the deluxe, they decide to remove the deluxe on that train.

The cost for the deluxe is just not competitive with flying in most markets, contributing to low ridership. I know on the "direct" T trains between Hong Kong and Shanghai or Beijing that the deluxe are often empty.