User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Other
User
HKCU\software\policies\microsoft\office\16.0\outlook\preferences
This policy allows you to manage the user setting for enabling or disabling automatic migration to the new Outlook app, initiated by Microsoft. If you don't configure this policy (default), the user setting for automatic migration is not controlled by the policy, allowing the user to manage it themselves. This user setting for automatic migration is enabled by default. If you enable this policy, the user setting controlling automatic migration is enabled. Automatic migration to the new Outlook app is allowed, and the user cannot change this setting. If you disable this policy, the user setting controlling automatic migration is disabled. Automatic migration to the new Outlook app is not allowed, and the user cannot change this setting. Note that this user setting is not available for migration initiated via the 'Admin-Controlled Migration to New Outlook' policy.