10 files declare their use of WebformElementEntityReferenceInterface in Webform 6.x

EntityAutocomplete.php in src/Plugin/WebformElement/EntityAutocomplete.php

OptionsBase.php in src/Plugin/WebformElement/OptionsBase.php

webform.tokens.inc in ./webform.tokens.inc

Builds placeholder replacement tokens for webforms and submissions.

WebformCompositeBase.php in src/Plugin/WebformElement/WebformCompositeBase.php

WebformDevelSchema.php in modules/webform_devel/src/WebformDevelSchema.php

WebformManagedFileBase.php in src/Plugin/WebformElement/WebformManagedFileBase.php

WebformSubmissionExportImportImporter.php in modules/webform_submission_export_import/src/WebformSubmissionExportImportImporter.php

WebformSubmissionForm.php in src/WebformSubmissionForm.php

WebformTermCheckboxes.php in src/Plugin/WebformElement/WebformTermCheckboxes.php

WebformTermSelect.php in src/Plugin/WebformElement/WebformTermSelect.php