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 (!this.pdfDocument) {
      return Promise.resolve(null);
    }
    if (!this._optionalContentConfigPromise) {
      
      
      return this.pdfDocument.getOptionalContentConfig();
    }
    return this._optionalContentConfigPromise;
  }
  /**
   * @param {Promise<OptionalContentConfig>} promise - A promise that is
   *   resolved with an {@link OptionalContentConfig} instance.
   */
  set optionalContentConfigPromise(promise) {