reverted:
--- b/core/modules/text/src/Plugin/Field/FieldType/TextWithSummaryItem.php.orig
+++ /dev/null
@@ -1,128 +0,0 @@
-<?php
-
-namespace Drupal\text\Plugin\Field\FieldType;
-
-use Drupal\Core\Field\FieldStorageDefinitionInterface;
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\TypedData\DataDefinition;
-use Drupal\Core\StringTranslation\TranslatableMarkup;
-
-/**
- * Plugin implementation of the 'text_with_summary' field type.
- *
- * @FieldType(
- *   id = "text_with_summary",
- *   label = @Translation("Text (formatted, long, with summary)"),
- *   description = @Translation("This field stores long text with a format and an optional summary."),
- *   category = @Translation("Text"),
- *   default_widget = "text_textarea_with_summary",
- *   default_formatter = "text_default"
- * )
- */
-class TextWithSummaryItem extends TextItemBase {
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function defaultFieldSettings() {
-    return [
-      'display_summary' => 0,
-      'required_summary' => FALSE,
-    ] + parent::defaultFieldSettings();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
-    $properties = parent::propertyDefinitions($field_definition);
-
-    $properties['summary'] = DataDefinition::create('string')
-      ->setLabel(new TranslatableMarkup('Summary'));
-
-    $properties['summary_processed'] = DataDefinition::create('string')
-      ->setLabel(new TranslatableMarkup('Processed summary'))
-      ->setDescription(new TranslatableMarkup('The summary text with the text format applied.'))
-      ->setComputed(TRUE)
-      ->setClass('\Drupal\text\TextProcessed')
-      ->setSetting('text source', 'summary');
-
-    return $properties;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function schema(FieldStorageDefinitionInterface $field_definition) {
-    return [
-      'columns' => [
-        'value' => [
-          'type' => 'text',
-          'size' => 'big',
-        ],
-        'summary' => [
-          'type' => 'text',
-          'size' => 'big',
-        ],
-        'format' => [
-          'type' => 'varchar_ascii',
-          'length' => 255,
-        ],
-      ],
-      'indexes' => [
-        'format' => ['format'],
-      ],
-    ];
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isEmpty() {
-    $value = $this->get('summary')->getValue();
-    return parent::isEmpty() && ($value === NULL || $value === '');
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function fieldSettingsForm(array $form, FormStateInterface $form_state) {
-    $element = [];
-    $settings = $this->getSettings();
-
-    $element['display_summary'] = [
-      '#type' => 'checkbox',
-      '#title' => $this->t('Summary input'),
-      '#default_value' => $settings['display_summary'],
-      '#description' => $this->t('This allows authors to input an explicit summary, to be displayed instead of the automatically trimmed text when using the "Summary or trimmed" display type.'),
-    ];
-
-    $element['required_summary'] = [
-      '#type' => 'checkbox',
-      '#title' => $this->t('Require summary'),
-      '#description' => $this->t('The summary will also be visible when marked as required.'),
-      '#default_value' => $settings['required_summary'],
-    ];
-
-    return $element;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getConstraints() {
-    $constraints = parent::getConstraints();
-    if ($this->getSetting('required_summary')) {
-      $manager = $this->getTypedDataManager()->getValidationConstraintManager();
-      $constraints[] = $manager->create('ComplexData', [
-        'summary' => [
-          'NotNull' => [
-            'message' => $this->t('The summary field is required for @name', ['@name' => $this->getFieldDefinition()->getLabel()]),
-          ],
-        ],
-      ]);
-    }
-    return $constraints;
-  }
-
-}
diff -u b/core/modules/text/tests/src/Functional/TextFieldTest.php b/core/modules/text/tests/src/Functional/TextFieldTest.php
--- b/core/modules/text/tests/src/Functional/TextFieldTest.php
+++ b/core/modules/text/tests/src/Functional/TextFieldTest.php
@@ -7,6 +7,7 @@
 use Drupal\field\Entity\FieldConfig;
 use Drupal\field\Entity\FieldStorageConfig;
 use Drupal\filter\Entity\FilterFormat;
+use Drupal\filter\Render\FilteredMarkup;
 use Drupal\Tests\field\Functional\String\StringFieldTest;
 use Drupal\Tests\TestFileCreationTrait;
 
@@ -22,9 +23,7 @@
   }
 
   /**
-   * Modules to enable.
-   *
-   * @var array
+   * {@inheritdoc}
    */
   protected static $modules = ['entity_test', 'file', 'field_ui'];
 
@@ -183,15 +182,31 @@
     $format2 = FilterFormat::create([
       'format' => mb_strtolower($this->randomMachineName()),
       'name' => $this->randomMachineName(),
+      'filters' => [
+        'filter_html' => [
+          'status' => 1,
+          'settings' => [
+            'allowed_html' => '<strong>',
+          ],
+        ],
+      ],
     ]);
     $format2->save();
 
+    // Create a third text format.
+    $format3 = FilterFormat::create([
+      'format' => mb_strtolower($this->randomMachineName()),
+      'name' => $this->randomMachineName(),
+    ]);
+    $format3->save();
+
     // Grant access to both formats to the user.
     $roles = $this->webUser->getRoles();
     $rid = $roles[0];
     user_role_grant_permissions($rid, [
       $format1->getPermissionName(),
       $format2->getPermissionName(),
+      $format3->getPermissionName(),
     ]);
 
     // Create a field with multiple formats allowed.
@@ -227,6 +242,24 @@
     $this->assertSession()->fieldExists("{$field_name}[0][value]", NULL);
     $this->assertSession()->optionExists("{$field_name}[0][format]", $format1->id());
     $this->assertSession()->optionExists("{$field_name}[0][format]", $format2->id());
+    $this->assertSession()->optionExists("{$field_name}[0][format]", $format3->id());
+
+    $filtered_markup = FilteredMarkup::create('<div><strong><span>Hello World</span></strong></div>');
+    $edit = [
+      "{$field_name}[0][value]" => $filtered_markup,
+    ];
+    $this->submitForm($edit, 'Save');
+    preg_match('|entity_test/manage/(\d+)|', $this->getUrl(), $match);
+    $id = $match[1];
+    $this->assertSession()->pageTextContains('entity_test ' . $id . ' has been created.');
+
+    // Display the entity.
+    $entity = EntityTest::load($id);
+    $display = $entity_display_repository->getViewDisplay($entity->getEntityTypeId(), $entity->bundle(), 'full');
+    $content = $display->build($entity);
+    $rendered_entity = \Drupal::service('renderer')->renderRoot($content);
+    $this->assertStringContainsString('<div><strong><span>', (string) $rendered_entity);
+
     // Log back in as admin.
     $this->drupalLogin($this->adminUser);
     // Change field to allow only one format.
@@ -241,6 +274,23 @@
     $this->assertSession()->fieldExists("{$field_name}[0][value]", NULL);
     $this->assertSession()->fieldNotExists("{$field_name}[0][format]");
 
+    // Retest the entity renders fine even though filter2 is disabled.
+    $entity = EntityTest::load($id);
+    $display = $entity_display_repository->getViewDisplay($entity->getEntityTypeId(), $entity->bundle(), 'full');
+    $content = $display->build($entity);
+    $rendered_entity = \Drupal::service('renderer')->renderRoot($content);
+    $this->assertStringContainsString('<div><strong><span>', (string) $rendered_entity);
+
+    // Test when 2 of 3 formats are selected.
+    $field->setSetting('allowed_formats', [$format1->id(), $format2->id()]);
+    $field->save();
+    $this->drupalGet('entity_test/add');
+    // We should see the 'format' selector again.
+    $this->assertSession()->fieldExists("{$field_name}[0][value]", NULL);
+    $this->assertSession()->optionExists("{$field_name}[0][format]", $format1->id());
+    $this->assertSession()->optionExists("{$field_name}[0][format]", $format2->id());
+    $this->assertSession()->optionNotExists("{$field_name}[0][format]", $format3->id());
+
     // Change field to allow all formats by configuring none as allowed.
     $field->setSetting('allowed_formats', []);
     $field->save();
@@ -249,6 +299,7 @@
     $this->assertSession()->fieldExists("{$field_name}[0][value]", NULL);
     $this->assertSession()->optionExists("{$field_name}[0][format]", $format1->id());
     $this->assertSession()->optionExists("{$field_name}[0][format]", $format2->id());
+    $this->assertSession()->optionExists("{$field_name}[0][format]", $format3->id());
   }
 
   /**
reverted:
--- b/core/modules/text/tests/src/Kernel/TextWithSummaryItemTest.php
+++ a/core/modules/text/tests/src/Kernel/TextWithSummaryItemTest.php
@@ -67,7 +67,7 @@
     $entity = $storage->create();
     $entity->summary_field->value = $value = $this->randomMachineName();
     $entity->summary_field->summary = $summary = $this->randomMachineName();
+    $entity->summary_field->format = NULL;
-    $entity->summary_field->format = 'plain_text';
     $entity->name->value = $this->randomMachineName();
     $entity->save();
 
@@ -76,7 +76,7 @@
     $this->assertInstanceOf(FieldItemInterface::class, $entity->summary_field[0]);
     $this->assertEquals($value, $entity->summary_field->value);
     $this->assertEquals($summary, $entity->summary_field->summary);
+    $this->assertNull($entity->summary_field->format);
-    $this->assertEquals('plain_text', $entity->summary_field->format);
     // Even if no format is given, if text processing is enabled, the default
     // format is used.
     $this->assertEquals("<p>{$value}</p>\n", $entity->summary_field->processed);
@@ -115,9 +115,6 @@
     $this->field = FieldConfig::create([
       'field_storage' => $this->fieldStorage,
       'bundle' => $entity_type,
-      'settings' => [
-        'allowed_formats' => ['plain_text'],
-      ],
     ]);
     $this->field->save();
   }
diff -u b/core/modules/text/text.post_update.php b/core/modules/text/text.post_update.php
--- b/core/modules/text/text.post_update.php
+++ b/core/modules/text/text.post_update.php
@@ -27,7 +27,7 @@
     ->update($sandbox, 'field_config', function (FieldConfigInterface $field_config) {
       $class = get_class($field_config);
       // Deal only with text fields and descendants.
-      if ($class == TextItemBase::class || is_subclass_of($class, TextItemBase::class)) {
+      if (is_a($class, TextItemBase::class, TRUE)) {
         // Get the existing allowed_formats setting.
         $allowed_formats = $field_config->get('settings.allowed_formats');
         if (!is_array($allowed_formats) && empty($allowed_formats)) {