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.
...this.documentInfo,
      baseURL: this.baseUrl,
      filesize: this._contentLength,
      filename: this._docFilename,
      metadata: this.metadata?.getRaw(),
      authors: this.metadata?.get("dc:creator"),
      numPages: this.pagesCount,
      URL: this.url,
    };
  },
  /**
   * A place to fetch data for telemetry after one page is rendered and the
   * viewer is idle.
   * @private
   */
  async _collectTelemetry(pdfDocument) {
    const markInfo = await this.pdfDocument.getMarkInfo();
    if (pdfDocument !== this.pdfDocument) {
      return; 
    }
    const tagged = markInfo?.Marked || false;
    this.externalServices.reportTelemetry({
      type: "tagged",
      tagged,
    });
  },
  /**
   * @private
   */
  async _initializeAutoPrint(pdfDocument, openActionPromise) {