`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.
for (const CTxIn& txin : tx.vin) {
        const Coin& coin = view.AccessCoin(txin.prevout);
        
        
        Assume(!coin.IsSpent());
        if (coin.IsSpent()) return false;
        
        
        
        
        const CTransactionRef& txFrom = pool.get(txin.prevout.hash);
        if (txFrom) {
            assert(txFrom->GetHash() == txin.prevout.hash);
            assert(txFrom->vout.size() > txin.prevout.n);
            assert(txFrom->vout[txin.prevout.n] == coin.out);
        } else {
            assert(std::addressof(::ChainstateActive().CoinsTip()) == std::addressof(coins_tip));
            const Coin& coinFromUTXOSet = coins_tip.AccessCoin(txin.prevout);
            assert(!coinFromUTXOSet.IsSpent());
            assert(coinFromUTXOSet.out == coin.out);
        }
    }
    
    return CheckInputScripts(tx, state, view, flags,  true,  true, txdata);
}
namespace {
class MemPoolAccept
{
public:
    explicit MemPoolAccept(CTxMemPool& mempool, CChainState& active_chainstate) : m_pool(mempool), m_view(&m_dummy), m_viewmempool(&active_chainstate.CoinsTip(), m_pool), m_active_chainstate(active_chainstate),