farm-2.x-dev/modules/core/ui/user/farm_ui_user.routing.yml

farm_ui_user.access_denied:
  path: '/403'
  defaults:
    _title: 'Access denied'
    _controller: 'Drupal\farm_ui_user\Controller\ErrorPageController::on403'
  requirements:
    # No access restriction for the 403 error page.
    _access: 'TRUE'