For Outlook 2016/365, the registry key/value that switches the Ribbon to Classic is this:
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Toolbars\Outlook]
"QuickAccessToolbarStyleExplorer"=dword:00000010
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences]
"EnableSingleLineRibbon"=dword:00000000
For Simplified:
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Toolbars\Outlook]
"QuickAccessToolbarStyleExplorer"=dword:00000014
[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Preferences]
"EnableSingleLineRibbon"=dword:00000001