score:4
Not necessarily complete answer with all the references:
Zhangjiakou-Jining line opened for traffic in 2011. Before that, the detour to Datong was necessary.
Many passenger trains gradually took advantage of the faster and more direct connections. As @mlc found, it probably happened in 2014-2016. After K3/4 was rerouted via Zhangjiakou-Jining line, Datong was not served by train.
In 2018, K3/4 was routed via Datong again with the timetable changes of April 10. However, no passenger service was provided at the station for the train. These changes included rerouting all the passenger trains between Jining and Zhangjiakou.
This is partially related to the construction of new HSR lines. The other, perhaps the more significant reason, is that the Z-J line also vital for China's energy needs: it connects Baotou, the centre of coal industry, to the rest of China to ensure industrial and residential needs of utmost importance. Z-J line was constructed primarily to be part of the "西部能源输送大通道" ("the Great Channel to delivery energy from the west"), passenger traffic is of secondary concern in face of energy challenges.
The timetable changes allowed the line to be used for heavy-duty freight (coal) trains with capacity > 10,000 tonnes.
Especially since a dedicated HSR passenger line is under construction, passenger traffic would be significantly relieved within two years but energy needs were (and are) much more urgent.