The New Outlook is Microsoft Outlook’s new interface. It’s the same application with a different appearance. Unfortunately, Microsoft also removed the support for AppleScript, an inter-application communication technology that DEVONthink and other apps need to talk to Outlook.
To enable email archiving from Outlook again, switch off the new appearance via Outlook > New Outlook. You’re presented with a prompt asking why you’re changing away from the new look. Please feel free to let Microsoft know by choosing AppleScript from the list of reasons.
According to Microsoft’s roadmap, AppleScript support is planned to return to Outlook.