6 files declare their use of OptGroup in Drupal 8

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

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

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

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

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

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