Why do standard OP_RETURN outputs also need to be IsPushOnly()?

In src/script/solver.cpp in the Bitcoin Core source we find the following snippet:

// Provably prunable, data-carrying output
//
// So long as script passes the IsUnspendable() test and all but the first
// byte passes the IsPushOnly() test we don't care what exactly is in the
// script.
if (scriptPubKey.size() >= 1 && scriptPubKey[0] == OP_RETURN && scriptPubKey.IsPushOnly(scriptPubKey.begin()+1)) {
    return TxoutType::NULL_DATA;
}

This is used by IsStandard() in policy.cpp along with -datacarriersize to determine if an OP_RETURN transaction is standard.

Why do we care about IsPushOnly()?

Note that the consensus code, before adding a coin to the UTXO set database, only checks IsUnspendable(), which only checks that the first byte is OP_RETURN.

/**
 * Returns whether the script is guaranteed to fail at execution,
 * regardless of the initial stack. This allows outputs to be pruned
 * instantly when entering the UTXO set.
 */
bool IsUnspendable() const
{
    return (size() > 0 && *begin() == OP_RETURN) || (size() > MAX_SCRIPT_SIZE);
}


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