Trim chicken and place in slow cooker. Season with salt, pepper, paprika, garlic, and onion powder.
Whisk barbecue sauce, broth, and brown sugar; pour over chicken. Add vinegar if using.
Cook on low 6–7 hours or high 3–4 hours, until chicken shreds easily.
Meanwhile, halve spaghetti squash, remove seeds, and roast cut-side down at 400°F for 30–40 minutes until tender.
Shred chicken with two forks in the slow cooker and mix with sauce.
Scrape squash into strands, divide onto plates, and top with pulled chicken. Add pickles or parsley to taste.