use

use Drupal\image\Entity\ImageStyle;

and load all image style with

$styles = ImageStyle::loadMultiple();

use

use Drupal\image\Entity\ImageStyle;

and load all image style with

$styles = ImageStyle::loadMultiple();

Also you can use entity_load_multiple with

 styles = \Drupal::entityTypeManager()->getStorage('image_style')->loadMultiple();