Windows 3.1

A Windows version from 1992 is saving Southwest's butt right now

Nearly every flight in the U.S. is grounded right now following a
CrowdStrike system update error that's affecting everything from
travel to mobile ordering at Starbucks -- but not Southwest Airlines
flights. Southwest is still flying high, unaffected by the outage
that's plaguing the world today, and that's apparently because it's
using Windows 3.1.

https://www.digitaltrends.com/computing/southwest-cloudstrike-windows-3-1/