Disable Prompts to migrate to use Outlook (new)

Q: Customer called about having to close 4 windows prompting them to migrate to Outlook (new).  

A: We uninstalled the Outlook (new) app from Installed Apps, created a new key in the registry, rebooted, and then Outlook (classic) seemed to open just fine.  Either manually create this key, or save the 4 lines to a file name DisableNewOutlook.reg and run it on the computer.

Hide the Try the new Outlook toggle in classic Outlook:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\General]
"HideNewOutlookToggle"=dword:00000001

https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/enable-disable-employee-access-new-outlook