The T-Mobile Galaxy S6/S6Edge is (strangely) the only variant that currently has an official Android 5.1.1 build, and it turns out there's an interesting little bonus hiding inside. Samsung has added support for RAW photography to the Galaxy S6 in this update. The catch is that it's not supported in the stock app at this time.
The Manual Camera testing app still reports that there's no RAW support, but installing a camera app like Camera FV-5 confirms that the full camera2 API is functioning (Snap Camera apparently works too). I'm not sure why Manual Camera isn't detecting it, but I've tested with Camera FV-5 and can confirm it's spitting out DNG RAW files along with JPEGs.
There's no option in the stock camera app to enable RAW capture, which seems odd. However, the T-Mobile update is out way ahead of the rest. I would imagine Samsung will release a RAW capture mode in the Samsung app store as a downloadable module, as it does with a number of other modes. In the meantime, RAW photography is a thing with the right apps.