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 (!(promise instanceof Promise)) {
      throw new Error(`Invalid optionalContentConfigPromise: ${promise}`);
    }
    if (!this.pdfDocument) {
      return;
    }
    if (!this._optionalContentConfigPromise) {
      
      
      return;
    }
    this._optionalContentConfigPromise = promise;
    const updateArgs = { optionalContentConfigPromise: promise };
    for (const pageView of this._pages) {
      pageView.update(updateArgs);
    }
    this.update();
    this.eventBus.dispatch("optionalcontentconfigchanged", {
      source: this,
      promise,
    });
  }
  /**
   * @type {number} One of the values in {ScrollMode}.
   */
  get scrollMode() {