src/JsonapiServiceProvider.php      | 1 +
 src/Normalizer/ScalarNormalizer.php | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/src/JsonapiServiceProvider.php b/src/JsonapiServiceProvider.php
index ec99547..f3ecce8 100644
--- a/src/JsonapiServiceProvider.php
+++ b/src/JsonapiServiceProvider.php
@@ -35,6 +35,7 @@ class JsonapiServiceProvider implements ServiceModifierInterface, ServiceProvide
     // @see \Drupal\jsonapi\Normalizer\ScalarNormalizer
     if (!$container->has('serializer.normalizer.primitive_data')) {
       $container->register('serializer.normalizer.scalar.jsonapi', ScalarNormalizer::class)
+        ->setDeprecated(TRUE, 'The "%service_id%" service is deprecated in JSON API 1.5, because Drupal 8.3.0 ships with the "serializer.normalizer.primitive_data" service. On Drupal 8.3.0 and newer sites, that service is used instead.')
         ->addTag('normalizer', ['priority' => 5]);
     }
   }
diff --git a/src/Normalizer/ScalarNormalizer.php b/src/Normalizer/ScalarNormalizer.php
index dd87afe..f190df3 100644
--- a/src/Normalizer/ScalarNormalizer.php
+++ b/src/Normalizer/ScalarNormalizer.php
@@ -6,6 +6,8 @@ use Drupal\jsonapi\Normalizer\Value\FieldItemNormalizerValue;
 use Drupal\jsonapi\Normalizer\Value\FieldNormalizerValue;
 use Symfony\Component\Serializer\Exception\UnexpectedValueException;
 
+@trigger_error(__NAMESPACE__ . '\ScalarNormalizer is deprecated in JSON API 1.5, because Drupal 8.3.0 ships with the "serializer.normalizer.primitive_data" service. On Drupal 8.3.0 and newer sites, that service is used instead.', E_USER_DEPRECATED);
+
 /**
  * @deprecated in favor of \Drupal\serialization\Normalizer\PrimitiveDataNormalizer, kept only for Drupal 8.2 sites using JSON API.
  * @internal