Pending revisions may become unavailable when untranslatable fields affect all translations

The approach here makes sense to me, I never understood how untranslatable fields could work with draft revisions and this essentially stops us having to worry any more.

One nit:

+++ b/core/modules/content_translation/src/ContentTranslationManager.php
@@ -158,8 +164,32 @@ protected function loadContentLanguageSettings($entity_type_id, $bundle) {
+    }
+
+    $result = FALSE;
+    foreach (Workflow::loadMultipleByType('content_moderation') as $workflow) {
+      /** @var \Drupal\content_moderation\Plugin\WorkflowType\ContentModeration $plugin */
+      $plugin = $workflow->getTypePlugin();
+      $entity_type_ids = array_flip($plugin->getEntityTypes());
+      if (isset($entity_type_ids[$entity_type_id])) {
+        if (!isset($bundle_id)) {
+          $result = TRUE;
+          break;
+        }
+        else {
+          $bundle_ids = array_flip($plugin->getBundlesForEntityType($entity_type_id));
+          if (isset($bundle_ids[$bundle_id])) {
+            $result = TRUE;
+            break;
+          }
+        }
+      }
+    }

I think two early return TRUE; instead of the assignment might make this more readable, with just return FALSE at the end.

Apart from the workaround this is a straightforward change, and we have an issue open to add the API that would allow us to remove the workaround at #2942907: Entity system does not provide an API for retrieving an entity variant that is safe for editing (or possibly a further follow-up to that issue).

Moving to RTBC.

Adding reviewer credit.

The approach here makes sense to me, I never understood how untranslatable fields could work with draft revisions and this essentially stops us having to worry any more.

I disagree. #2878556: Ensure that changes to untranslatable fields affect only one translation in pending revisions already solved the problem of changing UT fields in pending revisions. That issue introduced constraints to disallow that. This issue isn't about that: the s.t.r. only involve changing translatable fields in a pending revision, and UT fields in a default revision. At least in theory, that should be possible to work as expected without forcing the "Hide non translatable fields on translation forms" checkbox. However, given that this Critical bug exists with that checkbox off, I agree with implementing this patch as a stop-gap until having that checkbox unchecked can be made to work correctly without this data integrity bug.

Therefore, pushed to 8.6.x and 8.5.x.