SERVER-27543 describes how we created the OplogQueryMetadata with duplicated fields that were meant to move into OplogQueryMetadata from ReplSetMetadata. In 3.8 we can remove the duplicated fields from ReplSetMetadata as well as the lastOpVisible which no longer should have any users. As part of this work we will need to start processing the OplogQueryMetadata in the OplogFetcher to update our local state similar to what we currently do with the ReplSetMetadata. In 3.8 we can be confident that all OplogFetcher responses will contain the OplogQueryMetadata since it can only be in a mixed replica-set with 3.6, which knows how to return the OplogQueryMetadata.