`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.
{
    LOCK(cs_LastBlockFile);
    unsigned int nFile = fKnown ? pos.nFile : nLastBlockFile;
    if (vinfoBlockFile.size() <= nFile) {
        vinfoBlockFile.resize(nFile + 1);
    }
    bool finalize_undo = false;
    if (!fKnown) {
        while (vinfoBlockFile[nFile].nSize + nAddSize >= (gArgs.GetBoolArg("-fastprune", false) ? 0x10000  : MAX_BLOCKFILE_SIZE)) {
            
            
            
            assert(std::addressof(::ChainActive()) == std::addressof(active_chain));
            finalize_undo = (vinfoBlockFile[nFile].nHeightLast == (unsigned int)active_chain.Tip()->nHeight);
            nFile++;
            if (vinfoBlockFile.size() <= nFile) {
                vinfoBlockFile.resize(nFile + 1);
            }