Using classic Outlook for Windows in business environments
Sounded like a bill of goods from the MS rep.
Items in "Deleted" folder is automatically deleted in 10 days is the default norm. It's not new. If you delete an email, it goes into the "Deleted" folder.
There is no ways for an automated process (computer program) to know for sure 100% that an email is from a scammer. That's the reason why you see phishing emails coming into your inbox or junk folders then you delete them.
The issue is that after the email is reported as a Phishing email, it is moved to the Junk folder (not the Deleted item folder where the 10 days rule apply). The item should be removed as previous.