Magento 2 Site Maintenance Extension - 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 while your website is not live or undergoing maintenance. Magento 2 coming soon extension will enable you to notify visitors that a website is under maintenance. A fully customized solution to keep your visitors updated about the website with countdown timers, newsletters, social links and contact us options to put on the storefront when in maintenance mode.