ModerationHandler is both a base class and @internal

smustgrave → credited godotislate → .

This came up as a daily BSI target

Wasn't sure so posted in #core-development in slack

From @godotislate

@internal doesn't mean "can't use", it means "no BC support"

and the interface

**
 * Defines operations that need to vary by entity type.
 *
 * Much of the logic contained in this handler is an indication of flaws
 * in the Entity API that are insufficiently standardized between entity types.
 * Hopefully over time functionality can be removed from this interface.
 *
 * @internal
 */
interface ModerationHandlerInterface {

So believe this is a works as designed. Recommend we close this.

@John Franklin, How Drupal core uses the @internal tag is explained in API definition section → of the BC policy. In part, it states,

Internal APIs
Contributed and custom code should avoid calling internal APIs because they might change in minor releases and such changes may not be documented in change records.

That and the previous comment resolve the question. There is no bug here, and the documentation exists, I am changing this to a support request. Also, closing because it has been answered.