I think the criteria if a plugin/theme question is off-topic should be if it can reasonably be answered by the people actually participating in answering questions here. Featured plugins are almost by definition are both beta quality software which is in a flux of change, something that prevents people from actually being experts in them and they have very little users.
It will actually be more useful to answer yoast SEO, jetpack, w3tc and woocommerce questions, and there are probably more people that have expertise in them around here, then answering featured plugins questions, but I see the value of having a simple line drawn between what is on-topic and off-topic and I don't see the value of blurring this line for the featured plugins.
FWIW the json API in its current state is just a piece of bloat. It took me less time to implement a subset of the functionality that I need then the time I was willing to allocate to wade in that bloated code.
If that question would have been worded as "how do I create json API to retrieve a CPT" I could have answered it with a "built your own" solution, and someone else could have answered it with the json API plugin but as it is a bug report for the plugin it should be better handled in the plugin support channels in the same way I direct people to trac when they ask about bugs in wordpress.