Tony Redmond: "
When someone leaves an organization, a discussion often takes place about what to do with their mailbox and other data. For Exchange Online, the choice is straightforward:
Usually, the choice comes down to either a shared or inactive mailbox. Of course, the mailboxes belonging to ex-employees store other personal information in places like OneDrive for Business and Teams chat. Other information, like the documents kept in SharePoint Online sites, is by definition shared and remains accessible to other users. This discussion focuses on what to do about “leaver” mailboxes."