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I understand the Read/Unread control no longer shows in the message preview/reading pane in Legacy Outlook for Mac, and that area now shows a Settings button instead.
Please confirm whether you are using Legacy Outlook for Mac or New Outlook for Mac (toggle in Outlook).
In Legacy Outlook for Mac, it’s not possible to add or remove buttons on the ribbon, so restoring a removed built-in command to that same preview area isn’t supported. Reference: Customize views in Legacy Outlook for Mac
If you want the old one-click placement back, please kindly submit feedback from within Outlook: Help > Feedback (and for new Outlook for Mac, “Make a suggestion” is available). Reference: How do I give feedback on Microsoft 365?
Thank you for your understanding and time. Have a lovely day!