As part of adding a shard, setFeatureCompatibilityVersion is called on the new shard. setFCV was changed to allow writeConcern in v3.6, so when sending setFCV to the shard, the config server should use the writeConcern given for addShard to propagate the user's wTimeout. setFCV doesn't support writeConcern in v3.4 though, and because it is legal to add a v3.4 shard from a v3.6 config server, the CSRS can't propagate the writeConcern until the last_stable version is bumped to v3.6.