Microsoft plans to disable basic authentication for five Exchange Online connection protocols on October 13, 2020. They’ve been clear on this point for several months and are now moving to deliver tools and provide guidance about what people should do about clients that use basic auth connections with Exchange Web Services, Exchange ActiveSync, IMAP4, POP3, and Remote PowerShell. Work is needed to make sure that clients are prepared for the switchover to modern authentication.
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.