#936 to Mutianyu still available?

No, it exists, but won't directly take you to the wall. It is a little complicated to explain. But you can take the bus at Dongzhimen to Yujiayuan. Then transfer to another 936 bus to Mutianyu Huandao. Upon getting off, walk to the entrance of the wall.

For the direct way, you can take bus no. 867 bus that departs at 10:00 and 12:00 and get off at Mutianyuhuandao. Then head for the destination.