Apologies if Everybody Knows This already:
Nagging message "Outlook Closed While You Had Items Open".
(Quite recent, after an update? But I can't find it referred to here .. ..)
In Outlook 2019 there's a blank under "File > Options > General > Start up options* which prevents the User from managing this recent very intrusive StartUp message - which in any case is often quite wrong.
Use regedit to visit HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences and if it's not there already create the DWORD ReopenWindowsOption and set it to 1. The nag disappears. (Set to 0 the prompt will appear as described; set to 2 any previous unclosed windows are opened.)
Edition Windows 10 Pro
Version 22H2
Installed on ‎04/‎10/‎2020
OS build 19045.3208