User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > Calendar Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\options\calendar
This policy determines if Outlook can use REST to update calendars. If you enable this policy, Outlook will use REST to update supported Office 365 and Outlook.com calendars. If you disable this policy, Outlook won't use REST to update any calendars.