The BEST reason to keep a Hybrid Exchange Server
We are often asked about should I have a Hybrid Server (be that for administration only or for full Hybrid) and so last year we blogged TEN good reasons to keep a Hybrid Exchange Server.
We left out the best and most important one, which is that Microsoft do not support making changes to Exchange related attributes with anything other than Exchange PowerShell or the Exchange Admin Console (EAC). So using tools like ADSIEdit or third-party tools is simply not supported.
Please read this Technet article for more detail. The article talks about decommissioning Exchange from an on premise environment, but the same argument goes for any On Premise AD that is synced with AAD Connect to Office 365/Azure AD and where the Exchange Schema extensions have been installed or you find you have a need for them to control certain behaviour like hiding an object from the Global Address List.