I think `<frontend_model>` works for you as like `Magento1` 

    <field id="recipient_email" translate="label" type="text" sortOrder="10" showInDefault="1" showInWebsite="1" showInStore="1">
                        <label>Send Emails To</label>
                        <validate>validate-email</validate>
                        <frontend_model>YOUR_NAMESPACE\YOUR_MODULE\Block\System\Config\Form\Field\Disable</frontend_model>
                    </field>

and your `frontend_model` class should be

    <?php
    namespace YOUR_NAMESPACE\YOUR_MODULE\Block\System\Config\Form\Field;
    
    use Magento\Framework\Data\Form\Element\AbstractElement;
    
    class Disable extends \Magento\Config\Block\System\Config\Form\Field
    {    
        protected function _getElementHtml(AbstractElement $element)
        {
            $element->setDisabled('disabled');
            return $element->getElementHtml();
    
        }
    }

May be another method available for this,But right now it works fine for you.