To hide all folders on the user-specified (top) section of the Start menu, implement the Remove user's folders from the Start Menu Group Policy at User Configuration / AdministrativeTemplates / Start Menu & Taskbar.

When this policy is Enabled, The NoStartMenuSubFolders Value Name, a REG_DWORD data type, at HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer, has a data value of 1. When the policy is Disabled or Not configured, the data value is 0, or the NoStartMenuSubFolders Value Name is missing.