Magento 2 Maintenance Mode - Cynoinfotech
https://www.cynoinfotech.com/product/site-maintenance-for-magento-2/
The Magento 2 Coming Soon extension is a great engagement tool to keep your visitors interested when your website is not live or undergoing maintenance. It works by displaying a Coming Soon or Maintenance Mode page that informs visitors about the site's status. The Magento 2 Coming Soon extension allows store owners to display a custom page when the site is undergoing maintenance, upgrading, or a new product is coming soon, and to notify customers via email when the site is accessible.