Does SW do these every year? Last year we got burned badly due to the late notice.
Southwest has been implementing a seasonal baggage embargo for several years. The embargo is usually in effect from mid-November until early January. Cities included in 2018/19 were Belize City (BZE), Montego Bay, (MBJ), Mexico City (MEX), Nassau (NAS), and San Jose, Costa Rica (SJO). The embargo disallows any additional checked bags over the allowance of two per passenger, and prohibits overweight and oversize baggage and all boxes.
It's impossible to predict for sure if the embargo will return in 2019/20, but I think it's safe to assume it will. It will likely apply to at least the same cities.
You might want to try contacting Southwest directly (see Contact Us below for options) to ask specifically about 2019/20, but I'm not sure you'll receive a clear answer. If you do, please come back and let us know what you find out. Hopefully the info might help others avoid the disappointment from Southwest's late notification as you experienced last year.