`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.
LogPrintf("[%d%%]...", percentageDone); 
            reportDone = percentageDone/10;
        }
        uiInterface.ShowProgress(_("Verifying blocks...").translated, percentageDone, false);
        if (pindex->nHeight <= active_chainstate.m_chain.Height()-nCheckDepth)
            break;
        if (fPruneMode && !(pindex->nStatus & BLOCK_HAVE_DATA)) {
            
            LogPrintf("VerifyDB(): block verification stopping at height %d (pruning, no data)\n", pindex->nHeight);
            break;
        }
        CBlock block;
        
        if (!ReadBlockFromDisk(block, pindex, chainparams.GetConsensus()))
            return error("VerifyDB(): *** ReadBlockFromDisk failed at %d, hash=%s", pindex->nHeight, pindex->GetBlockHash().ToString());
        
        if (nCheckLevel >= 1 && !CheckBlock(block, state, chainparams.GetConsensus()))
            return error("%s: *** found bad block at %d, hash=%s (%s)\n", __func__,
                         pindex->nHeight, pindex->GetBlockHash().ToString(), state.ToString());
        
        if (nCheckLevel >= 2 && pindex) {
            CBlockUndo undo;
            if (!pindex->GetUndoPos().IsNull()) {
                if (!UndoReadFromDisk(undo, pindex)) {
                    return error("VerifyDB(): *** found bad undo data at %d, hash=%s\n", pindex->nHeight, pindex->GetBlockHash().ToString());
                }
            }
        }
        
        if (nCheckLevel >= 3 && (coins.DynamicMemoryUsage() + active_chainstate.CoinsTip().DynamicMemoryUsage()) <= active_chainstate.m_coinstip_cache_size_bytes) {