Here is the Web Client SDK 6.5 home page: vSphere Web Client SDK for vSphere 6.5 - VMware Developer Center
Please note that the download for SDK 6.5 contains both a flex-client-sdk directory (which is the Flex Client SDK 6.5, unchanged from 6.0) and an html-client-sdk, which is the only SDK you should be using from now on in order to be compatible with both the vSphere Web Client (Flex) and the vSphere Client (HTML) that are included with vSphere 6.5.
We will continue to release HTML Client SDK flings at vSphere HTML5 Web Client to go along with the vSphere Client on-going updates.