`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.
{
    assert(fPruneMode && nManualPruneHeight > 0);
    LOCK2(cs_main, cs_LastBlockFile);
    if (chain_tip_height < 0) {
        return;
    }
    
    unsigned int nLastBlockWeCanPrune = std::min((unsigned)nManualPruneHeight, chain_tip_height - MIN_BLOCKS_TO_KEEP);
    int count = 0;
    for (int fileNumber = 0; fileNumber < nLastBlockFile; fileNumber++) {
        if (vinfoBlockFile[fileNumber].nSize == 0 || vinfoBlockFile[fileNumber].nHeightLast > nLastBlockWeCanPrune) {
            continue;
        }
        PruneOneBlockFile(fileNumber);
        setFilesToPrune.insert(fileNumber);
        count++;
    }
    LogPrintf("Prune (Manual): prune_height=%d removed %d blk/rev pairs\n", nLastBlockWeCanPrune, count);
}
void PruneBlockFilesManual(CChainState& active_chainstate, int nManualPruneHeight)
{
    BlockValidationState state;
    const CChainParams& chainparams = Params();
    assert(std::addressof(::ChainstateActive()) == std::addressof(active_chainstate));
    if (!active_chainstate.FlushStateToDisk(
            chainparams, state, FlushStateMode::NONE, nManualPruneHeight)) {
        LogPrintf("%s: failed to flush state (%s)\n", __func__, state.ToString());
    }
}
void BlockManager::FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight, int chain_tip_height, int prune_height, bool is_ibd)