Regedit:

HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
DWORD: HideNewOutlookToggle
Value: 1

Source: https://answers.microsoft.com/en-us/outlook_com/forum/all/how-to-get-rid-of-new-tool-bar-saying-try-the-new/91cfddc8-a9b5-4195-9df2-7768d05501bd