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 { appConfig, externalServices } = this;
    const eventBus = externalServices.isInAutomation
      ? new AutomationEventBus()
      : new EventBus();
    this.eventBus = eventBus;
    this.overlayManager = new OverlayManager();
    const pdfRenderingQueue = new PDFRenderingQueue();
    pdfRenderingQueue.onIdle = this._cleanup.bind(this);
    this.pdfRenderingQueue = pdfRenderingQueue;
    const pdfLinkService = new PDFLinkService({
      eventBus,
      externalLinkTarget: AppOptions.get("externalLinkTarget"),
      externalLinkRel: AppOptions.get("externalLinkRel"),
      ignoreDestinationZoom: AppOptions.get("ignoreDestinationZoom"),
    });
    this.pdfLinkService = pdfLinkService;
    const downloadManager = externalServices.createDownloadManager();
    this.downloadManager = downloadManager;
    const findController = new PDFFindController({
      linkService: pdfLinkService,
      eventBus,
    });
    this.findController = findController;