In 4.2, we added a new postBatchResumeToken to each batch returned by a change stream, which superseded the older $_internalLatestOplogTimestamp mechanism used when merging the streams on mongoS. However, we had to retain the latter in 4.2 to facilitate upgrade from and downgrade to 4.0.

Now that it is no longer necessary, we should remove all remaining traces of $_internalLatestOplogTimestamp from the codebase.