A mongod with featureCompatibilityVersion 3.6 should have minimum outgoing wire version equal to its maximum outgoing wire version. This means that when it receives an isMaster response, it will close the connection if the response is not from a 3.6 (or higher) node.

A 3.6 mongod also needs to close connections to 3.4 mongods that it initiated when it goes into upgrading state.

This is will cause heartbeats to 3.4 replica set members to fail, so that the 3.4 member is not considered healthy. This will also prevent FCV=3.6 shards from establishing connections with 3.4 shards.