In 3.6 the MongoDB drivers added implicit support for sessions, which is triggered by the presence of the logicalSessionTimeoutMinutes field in the isMaster response.

In 3.6, MongoS will always set its internal FCV to 3.6, regardless of the FCV of the shards and as a result it will always report logicalSessionTimeoutMinutes in the isMaster response. This, combined with the drivers' spec means that the driver will always send session information, regardless of the FCV version of the shards.

Shards, which are at FCV 3.4 will reject any requests which contain session information and this essentially means that 3.6 drivers cannot be used to talk to a cluster which is at FCV 3.4.