If you wish to hide the New Outlook toggle or prevent the automatic migration, you can set a registry key. The specific key is:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
Create a new DWORD type entry with the name “HideNewOutlookToggle” and a value of “1“.
This will hide the “Try the new Outlook” toggle. Conversely setting it to 0 or deleting the value will re-enable it.