A radiator can burn plastic if the plastic is too close to the radiator.
Radiators for heating don't usually get hot enough to cause a fire, although plastic items that are too close to the radiators can warp or melt, especially if the plastic is in direct contact with them.
In some cases if the radiator gets hot enough it could potentially ignite.
Steam radiators are not as hot as electric radiator heaters so it's more likely for something to ignite if in direct contact with an electric radiator heater than a steam radiator.
Most steam radiator heaters often circulate the water between 160 F to 180 F and the radiator surface will be a bit cooler.
These temperatures are not likely to ignite most materials although the temperatures of the steam radiator can still melt plastics such as thin plastic and even cause the plastics to warp, especially if they are in close proximity or in direct contact with the radiator.
Electric radiator heaters reach higher temperatures and can easily melt plastic as well and even ignite curtains and other fabrics and upholstery.