The Multishipping\Plugin::beforeSave() method can be simplified by removing unused $subject argument

This can be simplified to:

/**
 * Map STEP_SELECT_ADDRESSES to Cart::CHECKOUT_STATE_BEGIN
 * @return void
 */
public function beforeSave()
{
	if ($this->checkoutSession->getCheckoutState() === State::STEP_SELECT_ADDRESSES) {
		$this->checkoutSession->setCheckoutState(\Magento\Checkout\Model\Session::CHECKOUT_STATE_BEGIN);
	}
}

A trick: a «before» plugin can avoid declaration of unused last arguments if it needs not modify any argument

See also:

• An example of a multishipping checkout with Stripe payment