Are you sure they are pursuing it? I hadn't heard anything about the OS for the last-gen devices. It seems like 4.6 will see the light of day with the 9000 and 8220 (and 8210 if it ever sees a release), 4.6.1 on the 8900 and 8930 (assuming that's the model number of Niagara), 4.7 for the 9500 and 9530, and 5.0 for the 8000 series devices. There's very minimal differences from each OS version, from what we have seen thus far, aside from the devices and radios they support.
With OS 5.0 increasing to 18.104.22.168 in the last couple weeks (when 22.214.171.124 was around a couple months ago), if there were other projects for the last-gen 8000 series devices, I would say they are finished by now (or perhaps the 4.6 project was simply renamed to 5.0, although we have known about 5.0 for a long, long time now). Perhaps the most astonishing thing would be that OS 4.5, which is just now becoming readily available through official major carrier channels after a year in development, may not see much in the way of future development (bug fixes, etc).
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