6 files declare their use of SearchIndexInterface in Drupal 8

HelpSearch.php in core/modules/help_topics/src/Plugin/Search/HelpSearch.php

NodeSearch.php in core/modules/node/src/Plugin/Search/NodeSearch.php

SearchMatchTest.php in core/modules/search/tests/src/Kernel/SearchMatchTest.php

SearchMultilingualEntityTest.php in core/modules/search/tests/src/Functional/SearchMultilingualEntityTest.php

SearchNodeUpdateAndDeletionTest.php in core/modules/search/tests/src/Functional/SearchNodeUpdateAndDeletionTest.php

SearchRankingTest.php in core/modules/search/tests/src/Functional/SearchRankingTest.php