The main reason for deprecating the Outlook client is that it does not have the capabilities that are expected from it with the latest releases of CRM. The Outlook Client is a COM addin, which are only supported in some versions of Microsoft Outlook (such as the desktop version). In the last few years, the Microsoft Office product group releases the Office add-ins feature, which does not rely on code that is installed on the user machine, so that results more reliable and easier to maintain.
The new add-ins can run on the Outlook Web App, Mobile Outlook and Outlook for Mac. With the new enhancements to Microsoft Dynamics 365, the July update and future releases, additional enhancements and improvements will be added to the Outlook addin, which is now called the Dynamics 365 App for Outlook.
In the upcoming release of Dynamics CRM (July 2017 update, or v9) additional enhancement were introduced tto the addin, which include more information that is available to the user in addition to tracking and viewing of additional information related to email recipients or regarding records. Users will also be able to search for records or navigate to records all from within the app.