6 files declare their use of FieldsHelperInterface in Search API 8

ContentEntity.php in src/Plugin/search_api/datasource/ContentEntity.php

IndexAddFieldsForm.php in src/Form/IndexAddFieldsForm.php

IndexFieldsForm.php in src/Form/IndexFieldsForm.php

ProcessorPluginBase.php in src/Processor/ProcessorPluginBase.php

SearchApiFieldTrait.php in src/Plugin/views/field/SearchApiFieldTrait.php

SoulMate.php in tests/search_api_test_extraction/src/Plugin/search_api/processor/SoulMate.php