They are still being made although not perhaps in this country. Slotted steel screws are all you can get in most developing countries - all made in China or India. I used to have to take pozidrive / prodrive screws back with me to Africa as I couldn't get them for love nor money. If the demand is there then I'm sure that companies like Axminster of Rutlands that buy in tools and consumables by the container load from the far east will start selling them. Beware though, they may look like steel but you have to treat them more like brass!
Steve
Steve