Exchange Online supports inactive mailboxes as a way to keep mailbox data online after Office 365 accounts are removed. Inactive mailboxes are available as long as a hold exists on them. You can update mailbox properties to exclude all or some org-wide holds. If you exclude holds from a mailbox, you run the risk that Exchange will permanently remove the mailbox. If that’s what you want, all is well, but if it’s not, then you might not be so happy.
Although Office 365 supervision policies are intended to monitor a subset of user communications, usually involving specific groups of people, you might want to use a policy to monitor all email. In that case, how do you make sure that your policy has everyone in scope? The problem is that supervision policies don’t support dynamic distribution lists, so you need to do some work to build and maintain a distribution list containing all user mailboxes.