Use paper towels to pat the pork shoulder dry. Place the shoulder into an 8.5-quart slow cooker. Add the onion and garlic. Stir in the apple cider vinegar, tomato paste, maple syrup, chili powder, and salt and pepper to taste.
Cover and cook for 8 to 10 hours on Low or until the meat is very tender and easily shreds.
Use two forks to pull the meat apart, then stir it into the cooking juices.