Preheat the oven to 400°F (200°C).
In a small bowl, combine salt, black pepper, smoked paprika, garlic powder, onion powder, and cayenne pepper.
Pat the chicken thighs dry with paper towels. Rub the olive oil evenly over the chicken thighs.
Sprinkle the spice mixture evenly over both sides of the chicken thighs, rubbing it in gently.
Place the chicken thighs on a baking sheet lined with parchment paper or a lightly greased baking dish, skin side up.
In a medium bowl, whisk together the tomato paste, apple cider vinegar, powdered erythritol, smoked paprika, garlic powder, onion powder, ground cumin, black pepper, salt, and water until smooth to make the keto BBQ sauce.
Brush half of the BBQ sauce generously over the chicken thighs.
Bake the chicken thighs in the preheated oven for 25 minutes.
Remove the chicken from the oven and brush the remaining BBQ sauce over the thighs.
Return the chicken to the oven and bake for an additional 15 minutes, or until the internal temperature reaches 165°F (74°C) and the skin is crispy.
Remove from the oven and let rest for 5 minutes before serving.