ds-8.x-3.9/tests/themes/ds_test_layout_theme/ds_test_layout_theme.layouts.yml

ds_test_layout_theme:
  label: 'DS Test Layout - Theme'
  category: 'Test Layout Theme'
  template: templates/ds-test-layout-theme
  class: '\Drupal\ds\Plugin\DsLayout'
  regions:
    ds_content:
      label: Content