Prepare the Spice Mix: In a small bowl, combine the allspice, oregano, salt, garlic powder, cinnamon, and cayenne pepper. Set aside.
Marinate the Chicken: In a shallow dish, mix the yogurt and lemon juice. Stir in half of the prepared spice mix (reserve the remaining spice mix for later). Add the chicken thighs to the dish, turning to coat them in the yogurt mixture. Cover and refrigerate for at least 4 hours or overnight for best results.
Cook the Chicken: Lightly spray the bottom of your slow cooker (5 or 6-quart size) with cooking spray. Place the marinated chicken and all of the yogurt mixture into the slow cooker. Cover and cook on HIGH for 2 hours or on LOW for 4 hours, until the chicken is tender and cooked through.
Shred the Chicken: Once cooked, remove the chicken from the slow cooker using a fork or slotted spoon (discard any excess liquid). Shred the chicken using two forks, then sprinkle the remaining spice mix over the top. Stir to coat the shredded chicken evenly.
Assemble and Serve: Warm the pita bread and fill each pocket with the shredded chicken. Add your choice of toppings like red onion, parsley, tomatoes, cucumbers, and a dollop of hummus.