I understand that when a user is disabled their one drive for business account is marked for deletion for 30 days and maintained in recycle bin for another 90 days. Access delegation can be configured to assign permission to the user's manager in such event.
I would like to understand if there is anything else that can be done to backup/retain the onedrive files of that users other than notification emails. I also would like understand how others are dealing with this scenario, have anyone implemented any archival mechanism to automatically archive files to a shared drive or move to another onedrive account? What would be the best recommended policies for disabled or deleted user accounts.
If your talking about just keeping the files instead of getting rid of them, you can extend and set in the onedrive admin center how long you keep onedrive (Sites) for even if the user is deleted.
This however is no a "Backup" you still would need to get a 3rd party solution if you want a longer than 90 day backup window. Since you can only rollback 90 days worth of deleted or changed items inside of OneDrive itself.
When the recycle bin period (91 days totally i believe) is over they're gone, if you havent set a retention policy like legal hold on the users OneDrive! Your license type determines if this is possible in your scenario..Otherwise, a third-party backup solution is your way to go