Microsoft has added a Try the new Outlook switch to classic Outlook for Windows. If you do not want to use the new version of Outlook, you can hide this switch with a simple Registry change.
First, close Outlook.
Press Windows Key + R, type regedit and press Enter. If Windows asks for permission to open Registry Editor, click Yes.
In Registry Editor, browse to:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
If the General folder is not there, right-click Options, choose New > Key, and name the new key General.
With General selected, right-click in the empty space on the right-hand side and choose New > DWORD (32-bit) Value.
Name the new value:
HideNewOutlookToggle
Double-click it, change the value from 0 to 1, then click OK.
Close Registry Editor and open Outlook again.
The Try the new Outlook switch should now be hidden.
How to Undo the Change
If you want the switch back later, return to the same Registry location:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General
Either delete HideNewOutlookToggle, or double-click it and change the value back to 0.
Restart Outlook and the switch should appear again.
What This Change Does
This only hides the Try the new Outlook switch in classic Outlook.
It does not uninstall the new Outlook application and does not stop it being installed separately.