radix-8.x-4.2/templates/block/block--system-main-block.html.twig

{#
/**
 * @file
 * Template for the main content block.
 */
#}
{% include '@radix/block/block.twig' with {
  html_tag: 'div',
  block_utility_classes: [
    'block--system-main',
  ],
} %}