Mix salsa and taco seasoning in slow cooker until blended. Top with chicken; cover with lid.
Cook on LOW 6 to 8 hours (or on HIGH 3 to 4 hours). Remove chicken from slow cooker, reserving sauce in slow cooker. Shred chicken with fork. Return to slow cooker; stir until chicken is evenly coated with sauce.
Bake taco shells as directed on package. Fill with chicken mixture; top with remaining ingredients.