Provide a TextLongFieldItemNormalizer with an option to return processed text.

modules/core/csv/farm_csv.services.yml

@@ -4,6 +4,10 @@ services:
     tags:
       - { name: normalizer, priority: 10 }
     arguments: ['@entity_type.manager', '@entity_type.repository', '@entity_field.manager']
+  farm_csv.normalizer.text_long_field_item:
+    class: Drupal\farm_csv\Normalizer\TextLongFieldItemNormalizer
+    tags:
+      - { name: normalizer, priority: 10 }
   farm_csv.normalizer.entity_reference_field_item:
     class: Drupal\farm_csv\Normalizer\EntityReferenceFieldItemNormalizer
     tags:

modules/core/csv/src/Normalizer/TextLongFieldItemNormalizer.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Drupal\farm_csv\Normalizer;
+
+use Drupal\serialization\Normalizer\FieldItemNormalizer;
+use Drupal\text\Plugin\Field\FieldType\TextLongItem;
+
+/**
+ * Normalizes long text fields for farmOS CSV exports.
+ */
+class TextLongFieldItemNormalizer extends FieldItemNormalizer {
+
+  /**
+   * The supported format.
+   */
+  const FORMAT = 'csv';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function normalize($field_item, $format = NULL, array $context = []): array|string|int|float|bool|\ArrayObject|NULL {
+    /** @var \Drupal\text\Plugin\Field\FieldType\TextLongItem $field_item */
+
+    // Return processed text, if desired.
+    if (!empty($context['processed_text'])) {
+      return $field_item->get('processed')->getValue();
+    }
+
+    // Delegate to the parent method.
+    return parent::normalize($field_item, $format, $context);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function supportsNormalization($data, string $format = NULL, array $context = []): bool {
+    return $data instanceof TextLongItem && $format == static::FORMAT;
+  }
+
+}