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.
https://office365itpros.com/2019/05/28/excluding-inactive-mailboxes-from-org-wide-retention-holds
