The mongo config server should not enable key generation until the featureCompatibilityVersion = 3.6 is set
The mongo should not write to oplog until featureCompatibilityVersion = 3.6 is set. For the sharded cluster the featureCompatibilityVersion is sent to mongos and it in turn send it to config servers and shards.

The existing multiversion test suite. https://github.com/mongodb/mongo/blob/master/jstests/multiVersion/upgrade_cluster.js
starts with last-stable binary = 3.4 and upgrades it to the latest = current master.

upgrade order is
config servers
shards
mongos

need to add a test case where
once the config servers are upgraded it verifies that

• config returns dummy signed $logicalTime and operation time
• mongod and mongos ignore afterClusterTime that can be sent by mongo shell

once mongod is upgraded

• mongod returns dummy signed $logicalTime and operation time
• mongod and mongos ignore afterClusterTime that can be sent by mongo shell

once mongos are upgraded and

• mongod and mongos returns dummy signed $logicalTime and operation time
• mongod and mongos ignore afterClusterTime that can be sent by mongo shell

once featureCompatibilityVersion = 3.6 is set

• mongos and mongod correctly process causally consistent requests

the downgrade should check this in reverse.