UpdateDriver::update() generates a "$v" field to indicate what version of the update language was used to apply the logged update, which is necessary for heterogeneous replica sets that may include nodes supporting multiple versions of the update language.

Any replica set that cannot include nodes with binVersion <= 3.6, however, can safely elide this field. Newer mongod versions will always apply updates with "kUpdateNode" semantics and will always assume "kUpdateNode" semantics from updates in the oplog.

Eliding the "$v" field will save some space in the oplog.