This comment from dylanb at https://github.com/w3c/html-aria/issues/67 makes a lot of sense to me.
It makes no sense to have a required attribute on a field that can only have unchecked and checked IF both values (unchecked and checked) are allowable. What is being required?
For a radio button grouping, the “required” attribute applies to the group and not to the individual items. It is impossible to select all the radio buttons in a group for example but it does make sense to have a grouping with no default value and expect the user to choose one value. This is equivalent to a required select with no default value.
For checkboxes, the only interpretation of required that makes sense is one where required is equivalent to saying: “thou shalt check this checkbox” (for example accepting license terms).
So this is my way of saying that aria-required makes sense on checkboxes but does not make sense on individual radio buttons.