@Override
  public Stream<BlockHeader> apply(final BlockHeader blockHeader) {
    long blockNumber = blockHeader.getNumber();
    if (isPostMergeBlock(blockNumber)) {
      return Stream.of(blockHeader);
    }
    storeBlockHeader(blockHeader);
    logProgress(blockHeader);
    return Stream.empty();
  }
A step in the synchronization process that saves historical block headers.
ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/sync/SavePreMergeHeadersStep.java