6 files declare their use of OptGroup in Zircon Profile 8

EntityReferenceItem.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php

Contains \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem.

InOperator.php in core/modules/views/src/Plugin/views/filter/InOperator.php

Contains \Drupal\views\Plugin\views\filter\InOperator.

ListItemBase.php in core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php

Contains \Drupal\options\Plugin\Field\FieldType\ListItemBase.

OptGroupTest.php in core/tests/Drupal/Tests/Core/Form/OptGroupTest.php

Contains \Drupal\Tests\Core\Form\OptGroupTest.

OptionsDefaultFormatter.php in core/modules/options/src/Plugin/Field/FieldFormatter/OptionsDefaultFormatter.php

Contains \Drupal\options\Plugin\Field\FieldFormatter\OptionsDefaultFormatter.

OptionsWidgetBase.php in core/lib/Drupal/Core/Field/Plugin/Field/FieldWidget/OptionsWidgetBase.php

Contains \Drupal\Core\Field\Plugin\Field\FieldWidget\OptionsWidgetBase.