Because Microsoft inexplicably refuses to enable multiple accounts in To-Do, I've been using To-Do for personal tasks and using Outlook tasks and SharePoint lists for professional tasks and projects. Not long ago, I deleted the completed tasks in my Outlook task pane for my Microsoft 365 user account, as well as tasks that had been migrated to my SPO list by a Flow.
Today, while I was in the M365 portal, I saw an icon for To-Do and opened it up to see what the web app was like. I was surprised to see completed tasks dating back more than a year. These had been deleted from Outlook. This implies that the content of the task module of Outlook is not the same as the content of To-Do.
What are the differences, i.e., why would a task survive in To-Do that had been deleted from Outlook?