> it is unusual for a prion disease to transmit from one species to another.
So I'd expect the risk to be higher when dealing with human remains.
There is a notable exception to this general rule:
> Creutzfeldt–Jakob disease, however, is thought to be caused by a prion that typically infects cattle, causing bovine spongiform encephalopathy and is transmitted through infected meat
The other problem (mentioned by another poster) is that just because someone didn't die of a prion disease doesn't mean they aren't harboring prions because the incubation period is 5 - 20 years.
General comment: the risk may very well end up being low, but I would personally prefer to avoid consuming plants that were grown on composted human brain or spinal material to lower my personal risk.
https://medlineplus.gov/ency/article/001379.htm
For CWD, You’re supposed to avoid eating anything that has come into contact with the brain, spine, or spinal fluid.
When livestock have an outbreak of a prion disease, they’re usually slaughtered and destroyed.
Searching for TSEs will garner you a lot of information if interested.