Spray the inside of a 6-quart slow cooker with cooking spray.
In the slow cooker, whisk together the ranch seasoning packet, chili powder, cumin, paprika, dried oregano, onion powder, garlic powder, salt, pepper, and chicken broth until combined.
Use tongs to add the chicken thighs to the seasoning mixture and turn them to coat evenly in the liquid, then arrange the thighs in a single layer smooth side down.
Cover and cook on high for 3 hours (up to 5) or on low for 4–6 hours, until the chicken shreds easily with a fork.
Turn off the slow cooker, remove the chicken, discard any excess fat, and shred the meat with two forks or dice it with a knife.
Return the shredded chicken to the slow cooker, stir to combine with the juices, taste, and adjust seasoning if needed.
Serve warm in charred corn or flour tortillas with your choice of toppings such as guacamole or avocado, lettuce, shredded cheddar, lime, cilantro, sour cream, or cilantro-lime sauce.