.../src/Kernel/FieldItemSerializationTest.php      | 62 ++++++++++++++++------
 1 file changed, 45 insertions(+), 17 deletions(-)

diff --git a/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php b/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php
index 0933bb4428..75cc0180e1 100644
--- a/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php
+++ b/core/modules/serialization/tests/src/Kernel/FieldItemSerializationTest.php
@@ -157,7 +157,7 @@ public function testFieldDenormalizeWithScalarValue() {
    * @param string $format
    *   The format to test. (NULL results in the format-agnostic normalization.)
    *
-   * @dataProvider providerTestCustomBooleanNormalization
+   * @dataProvider providerTestCustomBooleanDataTypeNormalization
    */
   public function testCustomBooleanNormalization(array $test_modules, $format) {
     // Asserts the entity contains the value we set.
@@ -198,46 +198,74 @@ public function testCustomBooleanNormalization(array $test_modules, $format) {
     $assert_denormalization($core_normalization);
   }
 
+  /**
+   * Tests a format-agnostic normalizer.
+   *
+   * @param string[] $test_modules
+   *   The test modules to install.
+   * @param string $format
+   *   The format to test. (NULL results in the format-agnostic normalization.)
+   *
+   * @group legacy
+   * @expectedDeprecation Drupal\test_fieldtype_boolean_emoji_normalizer\Normalizer\BooleanItemNormalizer is a normalizer for a @FieldType plugin. This is very strongly discouraged because it requires similar logic to be implemented for every normalization. Please write this as a normalizer for a @DataType plugin instead, then it works for the entire Drupal API-First ecosystem!
+   *
+   * @dataProvider providerTestCustomBooleanFieldTypeNormalization
+   */
+  public function testDeprecatedCustomBooleanFieldTypeLevelNormalization(array $test_modules, $format) {
+    $this->testCustomBooleanNormalization($test_modules, $format);
+  }
+
   /**
    * Data provider.
    *
    * @return array
    *   Test cases.
    */
-  public function providerTestCustomBooleanNormalization() {
+  public function providerTestCustomBooleanDataTypeNormalization() {
     return [
-      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the format-agnostic normalization' => [
-        ['test_fieldtype_boolean_emoji_normalizer'],
-        NULL,
-      ],
       'Format-agnostic @DataType-level normalizers SHOULD be able to affect the format-agnostic normalization' => [
         ['test_datatype_boolean_emoji_normalizer'],
         NULL,
       ],
-      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the JSON normalization' => [
-        ['test_fieldtype_boolean_emoji_normalizer'],
-        'json',
-      ],
       'Format-agnostic @DataType-level normalizers SHOULD be able to affect the JSON normalization' => [
         ['test_datatype_boolean_emoji_normalizer'],
         'json',
       ],
-      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the HAL+JSON normalization' => [
-        ['test_fieldtype_boolean_emoji_normalizer'],
+      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the HAL+JSON normalization' => [
+        ['test_datatype_boolean_emoji_normalizer', 'hal'],
         'hal_json',
       ],
-      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the HAL+JSON normalization' => [
+      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the XML normalization' => [
         ['test_datatype_boolean_emoji_normalizer', 'hal'],
+        'xml',
+      ],
+    ];
+  }
+
+  /**
+   * Data provider.
+   *
+   * @return array
+   *   Test cases.
+   */
+  public function providerTestCustomBooleanFieldTypeNormalization() {
+    return [
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the format-agnostic normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
+        NULL,
+      ],
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the JSON normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
+        'json',
+      ],
+      'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the HAL+JSON normalization' => [
+        ['test_fieldtype_boolean_emoji_normalizer'],
         'hal_json',
       ],
       'Format-agnostic @FieldType-level normalizers SHOULD be able to affect the XML normalization' => [
         ['test_fieldtype_boolean_emoji_normalizer'],
         'xml',
       ],
-      'Format-agnostic @DataType-level normalizers SHOULD be able to affect the XML normalization' => [
-        ['test_datatype_boolean_emoji_normalizer', 'hal'],
-        'xml',
-      ],
     ];
   }