reverted:
--- /dev/null
+++ a/src/DependencyInjection/Compiler/RemoveJsonapiFormatCompilerPass.php
@@ -0,0 +1,53 @@
+<?php
+
+namespace Drupal\jsonapi\DependencyInjection\Compiler;
+
+use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+
+/**
+ * Removes 'api_json' format from the 'serializer.formats' container parameter.
+ *
+ * We want the 'api_json' format to not be supported in the REST module. But the
+ * JSON API module also should not have to define al alternative 'serializer'
+ * service.
+ * This is achieved through removing the 'api_json' format from the
+ * 'serializer.formats' container parameter. The consequences of doing that:
+ *
+ * - the REST module no longer allows this format to be used
+ * - the 'serialization.exception.default' service does not support 'api_json',
+ *   hence a custom exception subscriber is needed, which this module has:
+ *   'jsonapi.exception_subscriber'
+ * - the 'serializer' service does support 'api_json'
+ *
+ * In other words: the 'serializer' service supports 'api_json', but nothing is
+ * aware of it. You could only know by calling 'serializer:supportsEncoding()'.
+ *
+ * @see \Drupal\serialization\RegisterSerializationClassesCompilerPass
+ * @see \Drupal\jsonapi\JsonapiServiceProvider::register()
+ * @see \Drupal\jsonapi\EventSubscriber\DefaultExceptionSubscriber
+ * @see \Drupal\Tests\jsonapi\Functional\RestJsonApiUnsupported
+ *
+ * @internal
+ */
+class RemoveJsonapiFormatCompilerPass implements CompilerPassInterface {
+
+  /**
+   * Updates the 'serializer.formats' container parameter.
+   *
+   * @param \Symfony\Component\DependencyInjection\ContainerBuilder $container
+   *   The container to process.
+   */
+  public function process(ContainerBuilder $container) {
+    if ($container->hasParameter('serializer.formats')) {
+      $filtered_formats = array_filter(
+        $container->getParameter('serializer.formats'),
+        function ($format) {
+          return $format !== 'api_json';
+        }
+      );
+      $container->setParameter('serializer.formats', array_values($filtered_formats));
+    }
+  }
+
+}
diff -u b/src/JsonapiServiceProvider.php b/src/JsonapiServiceProvider.php
--- b/src/JsonapiServiceProvider.php
+++ b/src/JsonapiServiceProvider.php
@@ -6,6 +6,8 @@
 use Drupal\Core\DependencyInjection\ServiceModifierInterface;
 use Drupal\Core\DependencyInjection\ServiceProviderInterface;
 use Drupal\jsonapi\DependencyInjection\Compiler\RegisterSerializationClassesCompilerPass;
+use Drupal\jsonapi\DependencyInjection\Compiler\RemoveJsonapiFormatCompilerPass;
+use Symfony\Component\DependencyInjection\Compiler\PassConfig;
 
 /**
  * Adds 'api_json' as known format and prevents its use in the REST module.
@@ -35,6 +37,7 @@
    */
   public function register(ContainerBuilder $container) {
     $container->addCompilerPass(new RegisterSerializationClassesCompilerPass());
+    $container->addCompilerPass(new RemoveJsonapiFormatCompilerPass(), PassConfig::TYPE_REMOVE);
   }
 
 }