No. Brackish is not preferred for goldfish. Goldfish are freshwater fish.
Brackish water has a salinity content between that of freshwater and saltwater. Specifically brackish water has a salinity between 1.001 and 1.022. Beyond that it is considered marine. Some brackish species can live in full marine, but very few can live in freshwater their whole lives.
Brackish isn't that difficult to do. Granted it does require one more step than the average freshwater tank, but it's not impossible. And certainly not as complicated as saltwater. I thought about doing a brackish setup before for a dragon goby and archer fish. Not a lot of plants for brackish, tho.