We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
if (!this._pagesCapability.settled) {
      return false;
    }
    
    
    return this._pages.every(function (pageView) {
      return pageView?.pdfPage;
    });
  }
  /**
   * @type {boolean}
   */
  get renderForms() {
    return this.#annotationMode === AnnotationMode.ENABLE_FORMS;
  }
  /**
   * @type {boolean}
   */
  get enableAnnotationEditor() {
    return !!this.#annotationEditorUIManager;
  }
  /**
   * @type {boolean}
   */
  get enableScripting() {