Season the chicken thighs evenly with salt and pepper on both sides. Set aside.
Heat the olive oil in a large skillet over medium heat. Add the onion, garlic, carrots, and celery, and sauté for 5 minutes until the vegetables are slightly softened.
Transfer the sautéed vegetables to the slow cooker. Place the seasoned chicken thighs on top.
Pour the chicken broth and soy sauce over the chicken and vegetables. Cover and cook on low for 6 hours or on high for 3 hours.
In a small bowl, whisk together the cornstarch and cold water until smooth. Stir the mixture into the slow cooker during the last 30 minutes of cooking to thicken the sauce.
Remove the chicken from the slow cooker and shred it into bite-sized pieces using two forks. Return the shredded chicken to the slow cooker and stir to combine with the sauce and vegetables.