Thank you for reaching us!
For your query, I understand that you are planning to decommission the Azure AD Connect server, and you would like to know what future impact post decommissioning.
Microsoft Entra Connect sync is essentially a one-way operation Where a copy of your on-premises Active Directory is transferred to Azure AD.
If you shut down or decommission the Microsoft Entra Connect server, it will stop syncing with on-premises Active Directory, it will not have any impact on the old data that was previously synced to Office 365, also that changes made in your on-premises Active Directory, such as adding or removing users or groups won’t be reflected in Microsoft Entra ID.
In the same server, if you have a setup like hybrid deployment of Exchange, SharePoint or Teams etc. where some of your users or resources are hosted on-premises and some are hosted in Office 365. In this situation it's essential to keep the Microsoft Entra Connect server up and running to ensure that the attributes and objects stay synchronized between both environments.
If you only wish to keep your existing information in Office 365 and have no need to update it with new or modified data from your local Active Directory, then decommissioning your Microsoft Entra Connect server might not make an impact on your scenario.
Before you decommissioning it is always good practices to have a backup of your Azure AD Connect configuration and database. This backup can be helpful in case you need to restore it at a later time.
I hope this answer helps! If you have any further questions, please feel free to ask.
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