Hiding non translatable fields with custom form displays not working

I think it's better suited as a custom solution and not something that should go into core.

Firstly, this kind of change introduces a lot of complexity. Core has to handle a wide variety of use cases, and enforcing this behavior across all entity forms and translations might create unintended side effects. For some workflows, users may actually need those non-translatable fields visible even during translation.

There are also performance concerns. Iterating through every field in a form to determine if it should be displayed or hidden based on translation settings adds overhead, particularly on complex forms with many fields. Making this a default behavior could negatively impact sites with complex content models or custom workflows that require all fields during translation.

Finally, Drupal core has to maintain flexibility, and this solution might limit that for users relying on more nuanced control. It could lead to breaking existing sites where those fields are expected to be available, potentially causing validation or data handling issues.

Example Code (Basically overriding core EntityFormBuilder in a custom module):

 public function getForm(EntityInterface $entity, $operation = 'default', array $form_state_additions = []) {
    $form = parent::getForm($entity, $operation, $form_state_additions);
  
    // Check if translation settings are enabled for the entity.
    // This allows us to manage multilingual content appropriately.
    if (\Drupal::service('content_translation.manager')->isEnabled($entity->getEntityTypeId(), $entity->bundle())) {
        $current_language = \Drupal::languageManager()->getCurrentLanguage();
  
        // Check if the entity has a translation for the current language.
        if ($entity->hasTranslation($current_language->getId())) {
            // Load the translated entity.
            // This allows us to modify the entity in the user's preferred language.
            $translated_entity = $entity->getTranslation($current_language->getId());
  
            // Get all translatable fields.
            // We need to differentiate translatable from non-translatable fields.
            $translatable_fields = array_filter($entity->getFieldDefinitions(), function ($field_definition) {
                return $field_definition->isTranslatable();
            });
  
            // This ensures the form only displays fields relevant to the current translation.
            foreach ($form as $field_name => &$field) {
                // Check if the field is defined in the entity and is non-translatable.
                if (isset($entity->getFieldDefinitions()[$field_name]) && !isset($translatable_fields[$field_name])) {
                    $field['#access'] = FALSE;
                }
            }
        }
    }
  
    return $form;
}