How to create a input tag have disable attribute in system.xml Magento 2

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();

    }
}

Keyur Shah's answer is correct but I found that it is not possible to copy the value of the disabled element. So instead of writing $element->setDisabled('disabled'); you can write $element->setData('readonly', 1);. This displays the field the same way but you can mark the value and copy it.

This is not supported by current XSD of system.xml (Magento/Config/etc/system.xsd).

To enable such behavior for all types of fields at once it is possible to extend \Magento\Config\Block\System\Config\Form\Field::render() (e.g. using plugins) to make it support new type value of attribute element:

<field id="token" ...>
      <label>Auth Token</label>
      <attribute type="disabled">1</attribute>
</field>

At the moment it is possible to specify any value for type of attribute element, but only value shared will be processed, see \Magento\Config\Block\System\Config\Form::_getSharedCssClass(). Similar processing can be added in a new plugin to disable element if such attribute is specified.