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.
const pdfScriptingManager = new PDFScriptingManager({
      eventBus,
      sandboxBundleSrc:
        typeof PDFJSDev === "undefined" ||
        PDFJSDev.test("!PRODUCTION || GENERIC || CHROME")
          ? AppOptions.get("sandboxBundleSrc")
          : null,
      scriptingFactory: externalServices,
      docPropertiesLookup: this._scriptingDocProperties.bind(this),
    });
    this.pdfScriptingManager = pdfScriptingManager;
    const container = appConfig.mainContainer,
      viewer = appConfig.viewerContainer;
    const pageColors = {
      background: AppOptions.get("pageColorsBackground"),
      foreground: AppOptions.get("pageColorsForeground"),
    };
    this.pdfViewer = new PDFViewer({
      container,
      viewer,
      eventBus,
      renderingQueue: pdfRenderingQueue,
      linkService: pdfLinkService,
      downloadManager,
      findController,
      scriptingManager:
        AppOptions.get("enableScripting") && pdfScriptingManager,
      renderer:
        typeof PDFJSDev === "undefined" ||
        PDFJSDev.test("!PRODUCTION || GENERIC")