11 files declare their use of PluginFormTrait in Search API 8

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

Basic.php in src/Plugin/search_api/tracker/Basic.php

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

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

FieldsProcessorPluginBase.php in src/Processor/FieldsProcessorPluginBase.php

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

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

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

TestBackend.php in tests/search_api_test/src/Plugin/search_api/backend/TestBackend.php

TestTracker.php in tests/search_api_test/src/Plugin/search_api/tracker/TestTracker.php

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