I had never heard of these either but came across something mentioned about them several weeks before my mother passed away. From what I understand, they are not necessarily the typical bed sore someone develops but more like organ failure. The skin is an organ. I can't remember where I was reading about them but you can find info if you google it. If I remember correctly, the typical place they develop is on the base of the back, but can develop on the feet and maybe other places. Typically show up anywhere from 2 days to several months before death. My mother developed one on the heel of one foot about 2 weeks before she died and then more showed up on the base of her back 2 days before she died. They come on very fast. Can appear as a blister or sore and within hours are open, terrible looking places. Hospice kept hers bandaged and tried to treat it to keep it from getting worse.
I haven't heard of such a thing and I hope whoever has will update and inform us. Did you by any chance attempt a try at the search engines, google or other for this?
https://www.healthline.com/health/kennedy-ulcer#causes