RealSesense developer kit cameras go through cycles of being in stock, out of stock and then in stock again. The SR300 is currently listed as being in stock in the Intel Click online store.
I would guess that the main reason for a camera being permanently withdrawn would be if there was a new model that did the same as the earlier camera and more as well. An example of this is how the F200 was withdrawn when the SR300, its next generation replacement with back-compatibility with F200 applications, was released.
Although I have no insight into Intel's release plans, if this pattern continues then I would expect the ZR300 to replace the R200 at some point as the camera of choice for outdoor / mobile applications.
In terms of new and near-future RealSense products, the Intel Euclid processing unit with a ZR300 camera inside it has just gone on sale in the Intel Click store, and a new R400 camera is being developed.