Forum Discussion
Remove On Premises exchange Hybrid and go fully Online
Hello,
Hope these links below add some value to this discussion.
How & When to De-commission Hybrid: https://docs.microsoft.com/en-us/exchange/decommission-on-premises-exchange
can't manage the attribute msExchHideFromAddressLists from Office Admin Panel.: https://social.technet.microsoft.com/Forums/office/en-US/89b424a2-85fa-4b6b-b3b2-71eae2455556/msexchhidefromaddresslists-azure-ad-synchronisation?forum=onlineservicesexchange
Hide from Address List in Dirsynced environment: https://www.tachytelic.net/2017/11/office-365-hide-a-user-from-gal-ad-sync/amp/
The only reason why Microsoft recommends to keep 1 CAS server/Exchange Hybrid (Free version) is to keep MSexch attributes intact. Once you de-commission Exchange, it removes those attributes from AD as well. However since you have AD Connect sync running the source of authority for the synced objects is on-premise & without those attributes it becomes hard to manage objects at times. Not everyone is a pro when it comes to modifying objects using ADSIedit or PS.
Deleted : Decommissioning the last exchange server should not revert the Schema and the msExchange Attributes are still available to be used. The only difference is that you wont have any Exchange Console to do that modification process and also to ensure that the Uniqueness of the SMTP is maintained as mentioned by somebody else earlier in this post.