The RedbearDuo is partially supported by the Particle toolset. However, the full toolset for the Duo is available on the Github for Redbear. The RedbearDuo toolset and the system firmware (aka DeviceOS) will not be updated, but any existing Duos will continue to function. I expect the toolset will continue to function as it is. They will just be stuck running an older version.
The heart of the RedbearDuo is a Particle P1 module, the same module that is the heart of the Photon. The STM32F microprocessor mentioned earlier is in the P1 module. As a result, some of the Particle toolset will function for the RedbearDuo.
I am using the RedbearDuo for my personal projects, and I have continued to purchase them after the official notice that they will no longer be supported. I would not continue for a commercial product, however.