User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\general
This policy setting controls whether the “Try the new Outlook” toggle is displayed in Outlook. If you enable this policy setting, the toggle for “Try the new Outlook” will be hidden and users will not have the ability to switch between the existing and new Outlook experiences. If you disable or do not configure this policy setting, the toggle for “Try the new Outlook” will be displayed. Note: This policy only applies to subscription-based Microsoft 365 Apps.