Our current workflow when dealing with departing employees is as follows:
1. Delete the user in On-Prem AD and run Azure AD sync.
2. Restore the deleted user in Office 365 (at this point it is a cloud only account).
3. Change the log in id from user@xyz.com to user@xyz.onmicrosoft.com
4. In On-Prem AD I create a disabled user in a Former Users OU and assign any aliases it may need.
5. I then re-link the restored user to the new disabled user (via mapping the new immutableid).
6. Once I run Azure AD sync again the account converts from cloud only to synced.
7. At this point I go into Exchange Online and convert the mailbox to shared and assign the permissions to the manager.
8. In Sharepoint Online I make the manager Site collection administrator of the OneDrive of the former user.
9. I then send an email to the manager instructing them how to access the shared mailbox and OneDrive. We typically give them 90 days after that we delete the account on-prem.
As you can see in this lengthy work flow we could greatly benefit from the this new automated workflow. Please make it happen for Hybrid customers.