User Configuration > Administrative Templates > Microsoft Office 2016 > Collaboration Settings
User
HKCU\software\policies\microsoft\office\16.0\common
This policy setting controls whether co-authors can use the chat functionality within an Office application to collaborate with each other when editing a document. If you enable or don’t configure this policy setting, users can chat with each other when co-authoring a document. If you disable this policy setting, users can’t chat with each other when co-authoring a document. Note: This policy setting only applies to subscription versions of Office, such as Microsoft 365 Apps for enterprise.