Calling ContentEntityStorageBase::doLoadMultipleRevisionsFieldItems() directly is deprecated in drupal:8.8.0 and the method will be made abstract in drupal:9.0.0. Storage implementations should override and implement their own loading logic.
ContentEntityStorageBase is an abstract class and it's implementation of doLoadMultipleRevisionsFieldItems() relies on the overriding classes implementation of the abstract protected function doLoadRevisionFieldItems(). ContentEntityStorageBase::doLoadRevisionFieldItems() is deprecated and will be removed in Drupal 9, at which time, the fallback logic in ContentEntityStorageBase::doLoadMultipleRevisionsFieldItems() will be removed and the method made abstract.
Before:
Your storage could implement loading logic in doLoadRevisionFieldItems() and allow the fallback implementation of doLoadMultipleRevisionsFieldItems() to call that method repeatedly.
Now:
You should override doLoadMultipleRevisionsFieldItems() and implement your loading logic there. You should deprecate and trigger an error from your doLoadRevisionFieldItems() method.
