Entity bundle classes can be defined and discovered using the Drupal\Core\Entity\Attribute\Bundle attribute

The Drupal\Core\Entity\Attribute\Bundle attribute can be added to EntityInterface classes to designate the class as a bundle class. In order for the bundle class to be automatically discovered, it must be in the \Entity namespace of a module and have the Bundle attribute.

The use of the attribute can be supplemental to, or in lieu of, defining bundle classes in entity_type_info or entity_type_info_alter hook implementations. The bundle class definitions discovered through attributes are added after entity_type_info implementations run and before entity_type_info_alter implementations. In the case attribute-defined bundle classes defined in multiple modules all target the same entity bundle, entity_type_info_alter implementations can be used to specify a different bundle class other than the one attribute discovery provides.

Usage
hook_entity_type_info()

  #[Hook('entity_bundle_info')]
  public function entityBundleInfo(): array {
    $bundles['some_entity']['bundle_class_a']['class'] = EntityTestBundleClassA::class;
    $bundles['some_entity']['bundle_class_a']['label'] = 'Bundle class A';
    $bundles['some_entity']['bundle_class_a']['translatable'] = TRUE;

    return $bundles;
  }

Attribute
Example of an using attribute override existing bundle class from hook_entity_bundle_info above:

#[Bundle(
  entityType: 'some_entity',
  bundle: 'bundle_class_a',
  label: new TranslatableMarkup('Bundle class A label set by attribute'),
  translatable: FALSE,
)]
class BundleClassOverrideA extends SomeEntity {}

The label and translatable properties are optional and will override the existing values only if the bundle properties have non-NULL values.

Example of creating a bundle class for entity types without a bundle entity type:

#[Bundle(
  entityType: 'some_entity',
  bundle: 'new_bundle',
  label: new TranslatableMarkup('New bundle'),
)]
class NewBundle extends SomeEntity {}

If creating a new bundle, setting the label explicitly is recommended, otherwise the bundle name will be used as the label.

Note that it is not possible to create a bundle class for a bundle (new_bundle in this example) for any entity type with a bundle entity type (such as node with node_type config entities). Bundle class definition for non-existing bundles will not be exposed as a new bundle. This is done to ensure that code that expects to be able to load the corresponding config entity for such a bundle to work consistently.

Such definitions are ignored instead of triggering an error to handle edge cases when adding or deleting those config entities.