If you have extensions that you have upgraded or that your partner has upgraded on your behalf, you will want to schedule a review for the following reason:
"Beginning with the Dynamics 365 (8.0) release, a new OData 4 endpoint was added to the Dynamics 365 application stack. This endpoint is known as the Web API. The Web API provides a development experience that can be used across a wide variety of programming languages, platforms, and devices. The Web API is intended to replace the CRM 2011 endpoint, also known as the SOAP endpoint, for accessing the Dynamics 365 organization web service and other provided web services."
"As of the release of Microsoft Dynamics CRM Online 2016 Update 1 and Microsoft Dynamics CRM 2016 Service Pack 1 (8.1.0), the CRM 2011 endpoint has been deprecated. The 2011 endpoint will be removed some time after the release of Dynamics 365 version 9. We plan to provide updates to the Dynamics 365 SDK assemblies and tools over the next several minor releases, retargeting them to use the Web API instead of the 2011 endpoint."
"We strongly encourage developers to use the Web API for accessing Dynamics 365 web services when writing new application code. You must update existing applications to use the Dynamics 365 Web API for accessing Dynamics 365 web services between now and the removal of the 2011 endpoint. When you develop plug-ins and custom workflow activities, continue to link those projects to the Dynamics 365 SDK assemblies. The Dynamics 365 SDK assemblies will be updated to use the Web API. This update will be fully transparent to you and any code written using the SDK itself will be supported."
"Additional details will be published in future SDK releases. For more information about the Web API, see Use the Microsoft Dynamics 365 Web API "