19 files declare their use of SolrBackendInterface in Search API Solr 4.x

BackendTrait.php in src/Controller/BackendTrait.php

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

IndexSolrMultisiteCloneForm.php in src/Form/IndexSolrMultisiteCloneForm.php

LocalActionAccessCheck.php in src/Access/LocalActionAccessCheck.php

SearchApiSolrAdminCommands.php in modules/search_api_solr_admin/src/Commands/SearchApiSolrAdminCommands.php

SearchApiSolrBackend.php in src/Plugin/search_api/backend/SearchApiSolrBackend.php

SearchApiSolrCommands.php in src/Commands/SearchApiSolrCommands.php

SearchApiSolrDevelCommands.php in modules/search_api_solr_devel/src/Commands/SearchApiSolrDevelCommands.php

SearchApiSolrTest.php in tests/src/Kernel/SearchApiSolrTest.php

search_api_solr.module in ./search_api_solr.module

SolrAdminAccessCheck.php in modules/search_api_solr_admin/src/Access/SolrAdminAccessCheck.php

SolrAdminCloudAccessCheck.php in modules/search_api_solr_admin/src/Access/SolrAdminCloudAccessCheck.php

SolrCommandHelper.php in src/Utility/SolrCommandHelper.php

SolrConfigSetController.php in src/Controller/SolrConfigSetController.php

SolrDocumentDeriver.php in src/Plugin/DataType/Deriver/SolrDocumentDeriver.php

SolrFieldType.php in src/Entity/SolrFieldType.php

SolrLegacyTest.php in modules/search_api_solr_legacy/tests/src/Kernel/SolrLegacyTest.php

StreamingExpressionBuilder.php in src/Utility/StreamingExpressionBuilder.php

Utility.php in src/Utility/Utility.php