`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.
{
    AssertLockHeld(cs_main);
    assert(std::addressof(g_chainman.m_blockman) == std::addressof(*this));
    
    
    for (const uint256& hash : locator.vHave) {
        CBlockIndex* pindex = LookupBlockIndex(hash);
        if (pindex) {
            if (chain.Contains(pindex))
                return pindex;
            if (pindex->GetAncestor(chain.Height()) == chain.Tip()) {
                return chain.Tip();
            }
        }
    }
    return chain.Genesis();
}
std::unique_ptr<CBlockTreeDB> pblocktree;
bool CheckInputScripts(const CTransaction& tx, TxValidationState& state,
                       const CCoinsViewCache& inputs, unsigned int flags, bool cacheSigStore,
                       bool cacheFullScriptStore, PrecomputedTransactionData& txdata,
                       std::vector<CScriptCheck>* pvChecks = nullptr)
                       EXCLUSIVE_LOCKS_REQUIRED(cs_main);
static FILE* OpenUndoFile(const FlatFilePos &pos, bool fReadOnly = false);
static FlatFileSeq BlockFileSeq();
static FlatFileSeq UndoFileSeq();
bool CheckFinalTx(const CBlockIndex* active_chain_tip, const CTransaction &tx, int flags)