To prepare for the removal of the CKEditor 4 module and all integrations with it, the Drupal.theme.mediaEmbedEditButton theme function was moved out of the Media module (out of the media/media_embed_ckeditor_theme asset library and the core/modules/media/js/media_embed_ckeditor.theme.js file) and into the CKEditor (4) module (into the ckeditor/drupal.ckeditor.plugins.drupalmedia asset library and the core/modules/ckeditor/js/ckeditor.drupalmedia.theme.js file).
The Drupal.theme.mediaEmbedPreviewError theme function remains in the Media module (in core/modules/media/js/media_embed_ckeditor.theme.js).
The only way this can affect you is if you were using Drupal.theme.mediaEmbedEditButton, which is extremely unlikely: this renders the button for the CKEditor 4 toolbar that is used specifically by the Drupal Media CKEditor 4 integration.
