Field Formatter only shows latest revision

Seems like a CORE issue.
Trying to fix in https://www.drupal.org/project/drupal/issues/3442267 🐛 Entity Reference Revision not loaded in EntityReferenceFormatterBase Active .

I'm not sure I agree that it's a core issue, particularly regarding EntityReferenceFormatterBase. This class is very generic. It's meant as a basis for all entity references including entities that are not revisionable as well is config entities. In order to "load a revision" you would have to actually check if the entity is revisionable, which is well outside the scope of that class and would add complication for extending classes.

I have done all that you've mentioned.

> It's meant as a basis for all entity references including entities that are not revisionable as well is config entities.
This point I can understand.
Anyways I have raised an MR.
Let's see what the core maintainers have to say about this.

@asherry could you please provide this as MR and add tests to ensure it works as expected, with and without revisioning enabled?

The current implementation also means that a parent prepareView() is never called, if not one of these two if clauses match!

So instead of:

+  /**
+   * {@inheritdoc}
+   */
+  public function prepareView(array $entities_items) {
+    $field_type = $this->fieldDefinition->getType();
+    if ($field_type === 'entity_reference') {
+      parent::prepareView($entities_items);
+    }
+    if ($field_type === 'entity_reference_revisions') {
+      // See \Drupal\entity_reference_revisions\Plugin\Field\FieldFormatter\EntityReferenceRevisionsFormatterBase::prepareView()
+      foreach ($entities_items as $items) {
+        foreach ($items as $item) {
+          if ($item->entity) {
+            $item->_loaded = TRUE;
+          }
+        }
+      }
+    }
+  }

shouldn't it be something like this:

+  /**
+   * {@inheritdoc}
+   */
+  public function prepareView(array $entities_items) {
+    $field_type = $this->fieldDefinition->getType();
+    if ($field_type === 'entity_reference_revisions') {
+      // See \Drupal\entity_reference_revisions\Plugin\Field\FieldFormatter\EntityReferenceRevisionsFormatterBase::prepareView()
+      foreach ($entities_items as $items) {
+        foreach ($items as $item) {
+          if ($item->entity) {
+            $item->_loaded = TRUE;
+          }
+        }
+      }
+    } else {
+        parent::prepareView($entities_items);
+    }
+  }

to be compatible with any parent implementation?

Any plans to finish this @asherry and @Vivek Panicker?