Lead paint doesn’t contain enough lead to significantly control radiation of any type. You need a sizable amount to block x-ray radiation: think about the thickness and weight of the vests radiation techs use as protection.
The lead is used as a pigment and helps to decrease dry time and to increase its durability, corrosion resistance, and fungicidal properties. Lead paint is quite cheap and is still used on outdoor structures like bridges, road markings, storage tanks, building exteriors, etc. Lead-free alternatives exist but aren’t always as durable or are comparably durable but often more expensive. There are no applications of which I am aware that require lead paint. It’s 100% a cost and convenience issue.
Only Nepal and the Philippines have enacted any meaningful control of industrial lead paint. The US reduced the allowed lead content about 15 years ago but lead based paint is still actively used.
P.s. I’m not just a crazy paint fanatic, I’m a paid, crazy paint fanatic - it’s part of my job. Welcome to the EPA in the United States - better than nothing, but still industry’s bitch.