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.
return this._pages.length;
  }
  getPageView(index) {
    return this._pages[index];
  }
  /**
   * @type {boolean} - True if all {PDFPageView} objects are initialized.
   */
  get pageViewsReady() {