Problem/Motivation
The 'checkboxes.html.twig' templates for core, stable, and classy all apply the form-checkboxes class, but only the classy 'radios.html.twig' template applies the form-radios class.
A theme inheriting from stable must override the radios template in order to apply styles in the same way as is possible with checkboxes. Individual checkbox and radio inputs receive the 'form-checkbox' and 'form-radio' classes, respectively (see \Drupal\Core\Render\Element\Checkbox::preRenderCheckbox and \Drupal\Core\Render\Element\Radio::preRenderRadio()).
Proposed resolution
Add the form-radios class to the radios.html.twig templates in core and stable for consistency.