A post by the Exchange development group tried to explain why mailboxes have SharePoint Online proxy addresses. It’s all down to the Microsoft 365 substrate, which needs the proxy addresses to ingest digital twins from SharePoint Online into Exchange Online for use by shared services like Microsoft Search. The upshot is that you can’t remove a mailbox permanently without some background processes kicking in to make sure that SharePoint is taken care of.
Planner now creates digital twins (copies) of tasks in user mailboxes in Exchange Online to make data available for eDiscovery and compliance. Storing items in the Microsoft 365 substrate is the same approach to making data available for search and compliance as taken by Teams and Yammer.