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.
return this._spreadMode;
  }
  /**
   * @param {number} mode - Group the pages in spreads, starting with odd- or
   *   even-number pages (unless `SpreadMode.NONE` is used).
   *   The constants from {SpreadMode} should be used.
   */
  set spreadMode(mode) {
    if (this._spreadMode === mode) {
      return; 
    }
    if (!isValidSpreadMode(mode)) {
      throw new Error(`Invalid spread mode: ${mode}`);
    }
    this._spreadMode = mode;
    this.eventBus.dispatch("spreadmodechanged", { source: this, mode });
    this._updateSpreadMode( this._currentPageNumber);
  }
  _updateSpreadMode(pageNumber = null) {