in #2861383: Rename i18n migrations to _translation The Drupal\migrate_drupal\Plugin\migrate\source\d6\i18nVariable source plugin was deprecated and replaced with Drupal\migrate_drupal\Plugin\migrate\source\d6\VariableTranslation. When this happened, the unit source test for i18nTranslation was copied for VariableTranslation, but the kernel source test at Drupal\Tests\migrate_drupal\Kernel\Plugin\migrate\source\d6\i18nVariableTest was not.
In #2791119: Write meaningful Migrate source tests It was decided that the unit tests for sources were insufficient and the kernel tests are preferred, so Variable Translation should have a kernel test extending Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase. The existing i18nTranslation test can likely be copied and renamed.
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Comment #2
quietone CreditAttribution: quietone as a volunteer commentedChange of plans and this is being done in #3004428: Remove references to i18nVariable, CommentType, CommentVariable and CommentVariablePerCommentType from the global deprecation listener list.. See #8 over there.