It's the return value of ShardRegistry::_exhaustiveFindOnConfig, which is called down the chain by CatalogManagerReplicaSet::_exhaustiveFindOnConfig, which extracts the ShardRegistry::QueryResponse::opTime and returns it as StatusWith<OpTimePair<vector<BSONObj>>>.

CatalogManagerReplicaSet::_exhaustiveFindOnConfig has a lot of callers, some of which have invariants on the returned opTime.