Before demote the last domain controller and remove the domain, you have to disable the synchronization between Azure AD and your on-premise domain by running the following command :
Set-MsolDirSyncEnabled -EnableDirsync $False
I invite you to read the following links to learn more about the command above:
Once the Directory synchronization is off , check if all synchronized accounts in Azure AD can be managed through Azure portal. If it's Okay , you can perform the demote of the last domain controller.
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