19 files declare their use of FieldInterface in Search API 8

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

AddURLProperty.php in src/Plugin/search_api/processor/Property/AddURLProperty.php

AggregatedFieldProperty.php in src/Plugin/search_api/processor/Property/AggregatedFieldProperty.php

ConfigurablePropertyBase.php in src/Processor/ConfigurablePropertyBase.php

ConfigurablePropertyInterface.php in src/Processor/ConfigurablePropertyInterface.php

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

FieldsHelper.php in src/Utility/FieldsHelper.php

FieldsHelperInterface.php in src/Utility/FieldsHelperInterface.php

FieldsProcessorPluginBase.php in src/Processor/FieldsProcessorPluginBase.php

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

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

Index.php in src/Entity/Index.php

IndexInterface.php in src/IndexInterface.php

RenderedItemProperty.php in src/Plugin/search_api/processor/Property/RenderedItemProperty.php

search_api.views.inc in ./search_api.views.inc

Views hook implementations for the Search API module.

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

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

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

UnsavedIndexConfiguration.php in src/UnsavedIndexConfiguration.php