Microsoft is developing a new ‘Edge Mode’ that lets users visit sites using the same rendering engine as Classic Edge to continue using legacy web applications. This mode is designed for organizations that need to utilize web apps that were designed for the legacy browser. Microsoft Edge Dev and Canary builds have also added a new option called ‘Open sites in Edge mode’ This feature is still being developed as can be seen by browser user-agent strings sent by Microsoft Edge, Classic Edge, and the IE and Edge modes.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-is-adding-classic-edge-mode-to-new-edge-browser/