Place the frozen chicken breasts in a single layer in the slow cooker. Sprinkle evenly with the salt, poultry seasoning, and black pepper. Dot the chicken with the thinly sliced butter.
Cover and cook on high for 4-6 hours—until the internal temperature of the thickest portion of each breast registers 165°F on a meat thermometer.
Once fully cooked, transfer the cooked chicken to a clean cutting board. Let rest for 5-10 minutes, then shred or slice as desired for use in recipes.