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.
if (params.has("disableautofetch")) {
      AppOptions.set(
        "disableAutoFetch",
        params.get("disableautofetch") === "true"
      );
    }
    if (params.has("disablefontface")) {
      AppOptions.set(
        "disableFontFace",
        params.get("disablefontface") === "true"
      );
    }
    if (params.has("disablehistory")) {
      AppOptions.set("disableHistory", params.get("disablehistory") === "true");
    }
    if (params.has("verbosity")) {
      AppOptions.set("verbosity", params.get("verbosity") | 0);
    }
    if (params.has("textlayer")) {
      switch (params.get("textlayer")) {
        case "off":
          AppOptions.set("textLayerMode", TextLayerMode.DISABLE);
          break;
        case "visible":
        case "shadow":
        case "hover":
          viewerContainer.classList.add(`textLayer-${params.get("textlayer")}`);
          try {
            await loadPDFBug(this);
            this._PDFBug.loadCSS();