In which developed countries is pinworm medicine over-the-counter?

USA for sure. I can only answer for the us. Pyrantel pamate has been available otc for decades under a variety of brand names. As one of the most regulated medication venues in the world, I would assume lesser regulated countries have similar products available, but I have never investigated its status elsewhere.
IN USA. In USA Pyrantel Pamoate and is also available in some other European countries.