Since there is currently no one (that I know of) at Alfresco whose primary responsibility (or one of them) is the management of the SDK as a project, there is no guess or official target date. Heck, there isn't even any specific target date for Alfresco 6 yet (talking about the GA or Enterprise releases). Maybe Ole Hejlskov can give some input - as far as I remember, he mentioned something with regards to the SDK at DevCon 2018 but since I don't use it I either did not really listen to that segment or just did not file that anywhere easily accessible in my memories...
Since the SDK is focused on building extensions, the Alfresco team for the core product has never been using the SDK to build those parts of the product. Only the teams working on addon products, like Governance Services and such, (may) have been using SDK before, though from what I hear that is also quite limited. This blog post seems to be an indication that when Alfresco teams do use SDK, they are (still) using an older version than SDK 3. As Angel Borroy has posted, it is possible to continue using SDK 3 even for Alfresco 6.0 with only a few changes.