This issue has been bothering me too for months. I made a setting change today and so far the Restart message has not come back.
Windows Settings
> Privacy
> Diagnostics and Feedback
--> Diagnostic Data: BASIC
**This is what I changed below**
--> View Diagnostic Data: TURN ON
it took me to App Store to download the software.
After restarting my computer and starting Outlook, Word and Excel, NO RESTART OFFICE MESSAGE !!!
I think what was happening is that the diagnostic data needed the app to save the information, even the BASIC only information. since I didn't have this app, it was causing the error.
Maybe this will help for you too if you didn't enable this feature.