Currently it is not possible to add a custom message to your checkout page via a store setting. However, with this JavaScript snippet in a Custom Tags (System > Modules > Custom Tags) section, you should be able to add a custom message of your choice.

if (window.location.href.indexOf("/checkout/") != -1) {
        $( document ).ready(function() {
            var subject = "Notice:"
            var msg = "We are experiencing shipping delays at this time due to COVID-19. We apologize for any inconveniences this may cause. If you have questions, please reach out to, and we will be happy to assist you further. Thank you for your patience and understanding, and stay safe and healthy."
            var bg_color = "#FF0000";
            var text_color = "#FFFFFF";
            var font_size = "1.5em";
            $("<div style='width:96%; padding:2%; margin:2% auto; background:" + bg_color +";color:"+text_color+"; text-align:center;font-size:" + font_size +";border-radius:30px;'><strong>" + subject + "</strong> " + msg + "</div>").insertBefore(".cart-body");

Feel free to modify the variables to reflect your message as well as font color, font size and background color.

This is how your code would appear in System > Modules > Custom Tags:

This is how it would look on your checkout page: