I work in an Enterprise where we have numerous pages that we provide access to from a WinForms application. These pages date back up to 15 years. They adhere to HTML4 and the Trident MSHTML engine. Rather than re-code these to simply be compliant, it is preferred to build a modern web app. However, we need to be sure our existing pages will continue to function while we create the new application.
IE mode on Microsoft Edge is a simplified experience that combines a modern rendering engine and compatibility with legacy sites that require Internet Explorer in a single browser. IE mode provides an integrated browsing experience in Microsoft Edge, using the integrated Chromium engine for modern sites and leveraging Internet Explorer 11 (IE11) for legacy sites that require the Trident MSHTML engine.
You could add your legacy pages and sites as a PWA (Progressive web apps), the new Edge insider lets you add any site as a PWA, whether they are actually a PWA such as Google Photos, Spotify web player, Outlook, Gmail etc or a simple article page.