"Outlook (New)" (aka "One Outlook") is React Native with a full Chromium webview, and the transition from out-of-the-box Windows-provided "Outlook Mobile" (Windows 8-10) to "Outlook (New)" has mostly completed, but the transition from Outlook (Classic) (aka "Bloated Corporate Outlook") seems somewhat stalled because a few big corporations have extensive plugins and cranky IT admins. (Also Outlook Mobile had a better than IE11 support and also has been Chromium on most platforms, it is "Outlook (Classic)" that continues to haunt us all with its ancient Word-based micro-browser fork of IE.)

Sometimes I really wish MSFT broke backwards compatibility in the places that matter, not places their corporate customers don’t care about.