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 moreInfoText = [
      this.l10n.get("error_version_info", {
        version: version || "?",
        build: build || "?",
      }),
    ];
    if (moreInfo) {
      moreInfoText.push(
        this.l10n.get("error_message", { message: moreInfo.message })
      );
      if (moreInfo.stack) {
        moreInfoText.push(
          this.l10n.get("error_stack", { stack: moreInfo.stack })
        );
      } else {
        if (moreInfo.filename) {
          moreInfoText.push(
            this.l10n.get("error_file", { file: moreInfo.filename })
          );
        }