6 files declare their use of OptGroup in Drupal 9
- 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