As several of you pointed out to us today, Qualcomm recently updated a blog post on its website regarding the "double-twist" camera launch gesture on the upcoming Nexus 5X and 6P. Specifically, they placed a strikethrough on the text identifying the feature, and clarified the reasoning for the change with the below information:
[UPDATE: The feature to launch the camera via double-twist of the device was replaced by double tap of the power key. The name Context Core has been changed to Android Sensor Hub.]
We've officially confirmed with Google this morning that the feature has indeed been dropped from the Nexus 5X and 6P. Google did not provide a particular explanation for the decision, but did say that they were initially trying both the twist gesture and the power button double-tap for the camera launch function, and ended up just going with the power button version.
The twist feature does have its fans, though I never found it especially intuitive on Moto devices, and I'm not the only one on the team with that opinion. Whether the feature can be re-enabled with custom ROMs or kernels, we'll just have to wait and see.