Hello insiders! Since Oct of last year, we released the ability to attach files from OneDrive, OneDrive for Business & Sharepoint to Insider Fast users. We have since then, been gathering feedback and monitoring telemetry to understand how the feature is being used, what works well, what doesn't & what's missing. Based on these data points, we have decided to remove this feature from Insiders Fast, on the 19th of April.
We are working on a much better version with better sharing support, and less restrictive permission controls, but we currently do not have an ETA on when this will be supported in Mac Outlook, but we hope to update you soon.
You will continue to see the updated UI with the new attachment well, but attachments from OneDrive, OneDrive for Business & SharePoint, will now show up in the body of the email as it used to before, and you will only be able to attach local files when composing an email.
If you have any issues, please do use Help > Contact Support to reach out to us and we will be happy to help!
Mac Outlook Team