User Configuration > Administrative Templates > Microsoft Outlook 2016 > Account Settings > Exchange > Cached Exchange Mode
User
HKCU\software\policies\microsoft\office\16.0\outlook\cached mode
By default, most shared folders that users access in other mailboxes are automatically downloaded and cached in the users' local OST files when Cached Exchange Mode is enabled. Only shared Mail folders are not cached. You can use this setting to change this behavior so that non-mail folders are not downloaded automatically.