In places where those exist they are definitely worth checking out (my opinion of course). We don't have either right here. The mining is essentially all placer mining so of no use. I can think of a couple mine shafts and one limestone cave across an area about 150 miles x 150 miles. What we do have .. in places .. are big rock slides / boulder fields. One way to dispose of a body in them is to hide it in a crack between boulders, then let nature (gravity) cover it with gravel over time. Another would be to dig into the talus, then rebury. Since this is mostly designated / legal wilderness, no heavy equipment would be available, and otherwise in areas simply too remote / too far from roads, it would be difficult for a human with mere muscle power to remove the rocks, etc that something with the assumed strength of a sasquatch could use to cover a grave / burial site.
I'm not as negative a that sounds, just being realistic about the challenges. It is very worth keeping an eye out for an opportunity. If the first ones don't pan out, another still might. We only need one success.