User Configuration > Administrative Templates > Microsoft PowerPoint 2016 > PowerPoint Options > Save
User
HKCU\software\policies\microsoft\office\16.0\powerpoint\options
This policy setting governs the default format for new presentation files that users create. If you enable this policy setting, when a user creates a new blank presentation, it is in the specified default format. Users may still override the default and specify a specific format when they create a presentation. If you disable or do not configure this policy setting, PowerPoint Presentation is the default option.