Place chicken in the slow cooker and top with soy sauce, vinegar, ketchup, brown sugar, garlic, ginger, red pepper flakes, and water chestnuts.
¼ cup low-sodium soy sauce, 2 tablespoons apple cider vinegar, 2 tablespoons ketchup, 1 tablespoon dark brown sugar, 1 tablespoon minced garlic, ½ teaspoon freshly grated ginger, ½ teaspoon crushed red pepper flakes, 8 ounces water chestnuts