`RewindBlockIndex` was made to allow nodes upgrading to new consensus rules post-segwit fork to 
download new consensus data (i.e witnesses) without having to go through a whole IBD. It has been pointed than now really few pre-segwit nodes care to upgrade and that this logic may be irrelevant.

See https://github.com/bitcoin/bitcoin/pull/17737#discussion_r358514335 in review  of #17737 for more context pointers.
break;
            }
            try {
                
                uint64_t nBlockPos = blkdat.GetPos();
                if (dbp)
                    dbp->nPos = nBlockPos;
                blkdat.SetLimit(nBlockPos + nSize);
                std::shared_ptr<CBlock> pblock = std::make_shared<CBlock>();
                CBlock& block = *pblock;
                blkdat >> block;
                nRewind = blkdat.GetPos();
                uint256 hash = block.GetHash();
                {
                    LOCK(cs_main);
                    
                    assert(std::addressof(g_chainman.m_blockman) == std::addressof(m_blockman));
                    if (hash != chainparams.GetConsensus().hashGenesisBlock && !m_blockman.LookupBlockIndex(block.hashPrevBlock)) {
                        LogPrint(BCLog::REINDEX, "%s: Out of order block %s, parent %s not known\n", __func__, hash.ToString(),
                                block.hashPrevBlock.ToString());
                        if (dbp)
                            mapBlocksUnknownParent.insert(std::make_pair(block.hashPrevBlock, *dbp));
                        continue;
                    }
                    
                    assert(std::addressof(g_chainman.m_blockman) == std::addressof(m_blockman));
                    CBlockIndex* pindex = m_blockman.LookupBlockIndex(hash);
                    if (!pindex || (pindex->nStatus & BLOCK_HAVE_DATA) == 0) {
                      BlockValidationState state;