DTOO131 - Trust Bar Notifications for unsigned application add-ins must be blocked.
This policy setting controls whether the specified Office application notifies users when unsigned application add-ins are loaded or silently disable such add-ins without notification. This policy setting only applies if you enable the 'Require that application add-ins are signed by Trusted Publisher' policy setting, which prevents users from changing this policy setting. If you enable this policy setting, applications automatically disable unsigned add-ins without informing users. If you disable this policy setting, if this application is configured to require that all add-ins be signed by a trusted publisher, any unsigned add-ins the application loads will be disabled and the application will display the Trust Bar at the top of the active window. The Trust Bar contains a message that informs users about the unsigned add-in. If you do not configure this policy setting, the disable behavior applies, and in addition, users can configure this requirement themselves in the 'Add-ins' category of the Trust Center for the application.
Set the policy value for User Configuration -> Administrative Templates -> Microsoft PowerPoint 2016 -> PowerPoint Options -> Security -> Trust Center 'Disable Trust Bar Notification for unsigned application add-ins and block them' to 'Enabled'.