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It's gone. With the focus being on "realtimeness" in the new render pipeline it's most likely not coming back, anyway.
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It's gone, but may return.
See this page for an explanation.
As you're probably aware, AE CC 2015 is the first step in a major re-architecture, separating the renderer from the UI for the first time in AE's 20+ year history. This is a big job and it's clearly still in progress. (That page I linked details a number of things that aren't quite there yet.) This re-architecture is a necessary step for improving After Effects performance, but it's taking a while (and multiple versions) for them to get through it. So, during this time of transition as they're introducing elements of the new architecture, sometimes it may be easier to use the older version to get a job done.
I have CC 2014 and CC 2015 installed on all of my machines at my home studio so I can bounce back and forth if need be (project files are interchangeable between the two versions). I prefer to work in CC 2015 because it's much snappier, has better expression handling, etc., but I usually render with CC 2014 since it has multiprocessing and CC 2015 doesn't have the replacement for that yet. If you don't have CC 2014 installed, it's very easy to install it alongside CC 2015. Information here.
If you'd like to see the Cache Work Area in the Background feature return sooner rather than later, file a feature request. In conversations I've had with folks on the AE team, feature requests make a big difference in weighting priorities. So, if you know other folks who miss this feature, have them file feature requests too.