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._pageLabels?.[this._currentPageNumber - 1] ?? null;
  }
  /**
   * @param {string} val - The page label.
   */
  set currentPageLabel(val) {