that's basically not possible if you want sso between onpremise and exchange online: https://learn.microsoft.com/en-us/exchange/decommission-on-p...

there are tools so that you do not need it, but well its not supported. in recent updates there is a new supported scenario, you can remove the exchange but you need to keep the schema and powershell modules and than you do everything with powershell

Yes, that's what I intend. To keep the schema and sync the objects via AAD connect or what ever microsoft wants to call AAD now.

Well there is aad connect and aad connect cloud sync. Btw I still don’t get it that they have no solution to remove the last exchange server. It can be so easy to make it work, but well…