I haven't seen one since HoW.

It no longer drops. There are at least a couple threads per day on this topic. Basically, the game code is broken. No one seems to have received the gun since HoW was released. I personally have run Fist of Crota a few hundred times during the past several weeks. One user reported having run it 600 times.

Only drops from sword of crota enemies but I'm not sure if they spawn during patrol anymore but it spawns during the first dark below mission

Yup