Does the deletion of Chromosome 20 cause immunity against prion disease?
I see one report of a viable autosomal monosomy, 

(https://www.nejm.org/doi/full/10.1056/NEJM196710122771502)

but in general, they are not survivable.

https://www.ncbi.nlm.nih.gov/pubmed/4086989
https://www.biology.iupui.edu/biocourses/N100/2k2humancsomaldisorders.html
https://www.ncbi.nlm.nih.gov/pubmed/29164644

A better question would be if person with a deletion or natural knockout of that gene are susceptible to that prion disease.  Those subjects likely exist in larger numbers, though it's likely that few have been exposed to any particular prion disease.