6 files declare their use of SourcePluginBase in Drupal 9

BlockTranslation.php in core/modules/block/src/Plugin/migrate/source/d6/BlockTranslation.php

CacheableEmbeddedDataSource.php in core/modules/migrate/tests/modules/migrate_cache_counts_test/src/Plugin/migrate/source/CacheableEmbeddedDataSource.php

ContentEntity.php in core/modules/migrate_drupal/src/Plugin/migrate/source/ContentEntity.php

MigrateExternalTranslatedTestSource.php in core/modules/migrate/tests/modules/migrate_external_translated_test/src/Plugin/migrate/source/MigrateExternalTranslatedTestSource.php

MigrateSourceTest.php in core/modules/migrate/tests/src/Unit/MigrateSourceTest.php

Contains \Drupal\Tests\migrate\Unit\MigrateSourceTest.

MigrationSourceCacheTest.php in core/modules/migrate/tests/src/Kernel/Plugin/source/MigrationSourceCacheTest.php