diff -u b/core/modules/migrate/src/Plugin/migrate/process/Migration.php b/core/modules/migrate/src/Plugin/migrate/process/Migration.php
--- b/core/modules/migrate/src/Plugin/migrate/process/Migration.php
+++ b/core/modules/migrate/src/Plugin/migrate/process/Migration.php
@@ -2,6 +2,9 @@
 
 namespace Drupal\migrate\Plugin\migrate\process;
 
+@trigger_error('The '. __NAMESPACE__ . '\Migration is deprecated in
+Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, use '. __NAMESPACE__ . '\MigrationLookup', E_USER_DEPRECATED);
+
 /**
  * Calculates the value of a property based on a previous migration.
  *
only in patch2:
unchanged:
--- a/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/MigrationTest.php
@@ -2,6 +2,9 @@
 
 namespace Drupal\Tests\migrate\Unit\process;
 
+@trigger_error('The '. __NAMESPACE__ . '\MigrationTest is deprecated in
+Drupal 8.4.0 and will be removed before Drupal 9.0.0. Instead, use '. __NAMESPACE__ . '\MigrationLookupTest', E_USER_DEPRECATED);
+
 use Drupal\Core\Entity\EntityStorageInterface;
 use Drupal\migrate\MigrateSkipProcessException;
 use Drupal\migrate\Plugin\MigrationInterface;
@@ -14,6 +17,9 @@
 use Prophecy\Argument;
 
 /**
+ * @deprecated in Drupal 8.4.x, to be removed before Drupal 9.0.x. Use
+ *   \Drupal\Tests\migrate\Unit\processMigrationLookupTest instead.
+ *
  * @coversDefaultClass \Drupal\migrate\Plugin\migrate\process\Migration
  * @group migrate
  */