At this time, Backupify does not back up Archived Mailboxes. The feature of enabling/disabling archived mailboxes was removed from the platform.
But, we do indeed backup Archived mail data. The reason being is that Office 365 adds archived mail data as a separate folder on their mailboxes (see here for more information on that).
Since we back up everything found in a users Office 365 mailbox, it would include this information. So, while we don't announce it as a feature anymore, we would indeed back up this information.