There have been multiple, very limited private previews for the feature but, for a variety of reasons, the feature has not progressed out of private preview. Also, being in private preview does not imply anything about when or if we will ever deliver the feature or what form the feature will take when do finally progress it and make it more broadly available. The only implication you can draw from a feature being in private preview is that it's something we are working on and have solicited the help of "some" customers for their feedback based on some limited use in their environment.