During initial sync, a replica clones all collections, while at the same time buffer its sync source's new oplog entries. Once cloning completes, the replica sets its "minValid" timestamp to the latest oplog entry, and replays the oplog up to minValid. At this point the replica has constructed a consistent snapshot of the data, and it can transition to a secondary.

In MMAPv1 days, it was possible to miss a document while cloning a collection, even if the document existed during the whole time we were cloning, because the document was moved during the clone. Therefore, during recovery or initial sync, when a replica would apply an update operation that referred to a missing document (either from a regular update or the "applyOps" command) it would attempt to fetch the document from the sync source, in case the document had been missed during cloning. If the document was found, the replica would advance its minValid timestamp to the sync source's current opTime, setting back its progress toward achieving a consistent snapshot.

In MongoDB 4.3 we no longer support MMAPv1, and we don't even support syncing from a server that uses MMAPv1. Post-MMAPv1 we can't miss documents during collection clone, so it is no longer necessary to fetch missing documents, nor to advance minValid. If an update refers to a missing document, then there must be a later oplog entry that deletes the document, so the replica can skip the update operation.

We don't have much documentation of this process, so not much has to change. replSetGetStatus.initialSyncStatus.fetchedMissingDocs field is removed, see two references in replSetGetStatus.txt and one in getLog.txt. There may be other places that need updating that I have no found, however!