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.