Windows 3.1, launched in 1992, is likely not getting any updates. So, when CrowdStrike pushed the faulty update to all its customers, Southwest wasn’t affected (because it didn’t receive an update to begin with).
Aside from Windows 3.1, Southwest also uses Windows 95 for its staff scheduling system.
One X user suggested that the company switch to Windows XP—it’s also no longer updated, and it can run Windows 3.1 applications via compatibility mode.
To develop a custom solution for an incredibly niche base seems like it’d be a waste of resources to CrowdStrike to me