@leavittjk This would have some truth. SWA just recently put out their updated schedule within the last month, (I believe) When they update they never update for more than several months, (really not sure the exact number) but never for an entire year. But with the return of the MAX I'm hoping an updated schedule will be presented.
@leavittjk here is actual information I found that is definitive, not my speculation.
Southwest Flight Schedule Now Bookable into August 2021!
Check Flight Schedules - Southwest Airlines