8 files declare their use of WebformElementDisplayOnInterface in Webform 6.x

View.php in src/Plugin/WebformElement/View.php

WebformAttachmentBase.php in modules/webform_attachment/src/Plugin/WebformElement/WebformAttachmentBase.php

WebformComputedBase.php in src/Plugin/WebformElement/WebformComputedBase.php

WebformDisplayOnTrait.php in src/Plugin/WebformElement/WebformDisplayOnTrait.php

WebformHorizontalRule.php in src/Plugin/WebformElement/WebformHorizontalRule.php

WebformMarkupBase.php in src/Plugin/WebformElement/WebformMarkupBase.php

WebformMore.php in src/Plugin/WebformElement/WebformMore.php

WebformVariant.php in src/Plugin/WebformElement/WebformVariant.php