Leaking 70 Year Old Drain Cap
Cast pipe and fittings do rust. And when it rusts on a threaded fitting that s not opened for a long while it is basically frozen.
Tapping GENTLY around and on the plug while wrenching it open may work.
Sometimes wrenching it tight and than loose free's the plug.
Heating the hub (outside section where the plug screws into) may help. Don't strike the cast when heating it.
If you find a rubber (hub-less) connector that fits over the plug you can drill, saw, cut the plug out install the new connector and cap.