19 files declare their use of ItemInterface in Search API 8

AddURL.php in src/Plugin/search_api/processor/AddURL.php

AggregatedFields.php in src/Plugin/search_api/processor/AggregatedFields.php

AggregatedFieldsTest.php in tests/src/Unit/Processor/AggregatedFieldsTest.php

BackendPluginBase.php in src/Backend/BackendPluginBase.php

BackendTest.php in modules/search_api_db/tests/src/Kernel/BackendTest.php

ContentAccess.php in src/Plugin/search_api/processor/ContentAccess.php

Database.php in modules/search_api_db/src/Plugin/search_api/backend/Database.php

HighlightTest.php in tests/src/Unit/Processor/HighlightTest.php

LanguageWithFallback.php in src/Plugin/search_api/processor/LanguageWithFallback.php

ProcessorInterface.php in src/Processor/ProcessorInterface.php

ProcessorPluginBase.php in src/Processor/ProcessorPluginBase.php

RenderedItem.php in src/Plugin/search_api/processor/RenderedItem.php

ResultSet.php in src/Query/ResultSet.php

ResultSetInterface.php in src/Query/ResultSetInterface.php

ReverseEntityReferences.php in src/Plugin/search_api/processor/ReverseEntityReferences.php

RoleAccess.php in src/Plugin/search_api/processor/RoleAccess.php

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

StemmerTest.php in tests/src/Unit/Processor/StemmerTest.php

ViewsPropertyExtractionTest.php in tests/src/Kernel/Views/ViewsPropertyExtractionTest.php