Southwest has posted a "Return to service" date of 03/11/2021 for placing the 737 MAX back into their fleet. If you are concerned about flying on the 737 MAX, you may want to check the aircraft scheduled to operate on your flights. The aircraft will show up when opening up the "Details" section of your confirmed reservation on the website or by searching for new flights on the website. Reminder - the 737-800 is not the same as the 737 MAX.
I believe that Southwest previously stated that they will work with customers on previously-booked travel not originally scheduled to operate with the 737 MAX, who are concerned that their flight was subsequently changed to the MAX. Of course, there always is the disclaimer that the aircraft can be changed at any time, due to weather, mechanical, or scheduling issues.
Quote from Southwest CEO Gary Kelly:
It will have been one year since the 737 MAX was pulled from service. Southwest is planning on taking delivery of 35 new 737 MAX aircraft through the end of this year.