Place the beef bones in a large pot and cover with water. Add apple cider vinegar and let sit for 30 minutes (this helps extract nutrients from the bones)., Bring the mixture to a boil, then reduce heat to low and simmer for 8-12 hours. Skim off any foam that rises to the surface., After simmering, remove the bones and strain the broth., In a separate pot, add the chopped carrots, celery, and pumpkin to the broth and simmer until the vegetables are soft (about 15 minutes)., Let the stew cool before serving. Store leftovers in the fridge for up to 5 days, or freeze for longer storage.