magento2/app/code/Magento/AdminNotification/etc/adminhtml/system.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Config:etc/system_file.xsd">
    <system>
        <section id="system">
            <group id="adminnotification" translate="label" type="text" sortOrder="250" showInDefault="1">
                <label>Notifications</label>
                <field id="use_https" translate="label" type="select" sortOrder="1" showInDefault="1" canRestore="1">
                    <label>Use HTTPS to Get Feed</label>
                    <source_model>Magento\Config\Model\Config\Source\Yesno</source_model>
                </field>
                <field id="frequency" translate="label" type="select" sortOrder="2" showInDefault="1" canRestore="1">
                    <label>Update Frequency</label>
                    <source_model>Magento\AdminNotification\Model\Config\Source\Frequency</source_model>
                </field>
                <field id="last_update" translate="label" type="label" sortOrder="3" showInDefault="1">
                    <label>Last Update</label>
                    <frontend_model>Magento\Config\Block\System\Config\Form\Field\Notification</frontend_model>
                </field>
            </group>
        </section>
    </system>
</config>