The API to enable and disable Fusion in Azure Sentinel is going to be deprecated. We are making it easier to configure with an option in the UI, and it will be turned ON by default.
Fusion uses graph-powered machine learning algorithms to correlate between millions of lower fidelity anomalous activities from different products such as Azure AD Identity Protection, and Microsoft Cloud App Security, to combine them into a manageable number of interesting security incidents. To help you prepare for this change, learn more about Fusion in Azure Sentinel: Reducing security alert fatigue using machine learning in Azure Sentinel.