User Configuration > Administrative Templates > Microsoft Outlook 2016 > Outlook Options > Preferences > E-mail Options
User
HKCU\software\policies\microsoft\office\16.0\outlook\setup
This policy setting allows you to prevent the use of Conversation arrangement in Views. If you enable this policy setting, you will prevent Conversational arrangement in Views. If you disable or do not configure this policy setting, users will be able to use Conversation arrangement in Views.