Miscellaneous topics that do not fit into specific categories.
If that key doesn't work, use HideNewOutlookToggle - when the toggle is hidden, Microsoft won't enable new Outlook. (And you can't accidentally hit the slider.)
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General DWORD: HideNewOutlookToggle Value: 1 https://www.slipstick.com/outlook/classic-outlook-not-new-outlook/
As an FYI, many Outlook keys don't exist until you either change a setting in Options or add the key yourself. Since there is no option in Outlook for these settings, you need to add the key.