The Sabre reservation system was created by IBM for American Airlines in the 1950s — when a computer filled an entire building floor and was built out of vacuum tubes — and remains actively used today by multiple airlines. The programming language has been switched several times over the past 60+ years, but essential compatibility remains.
https://www.theregister.com/2022/12/28/southwest_outdated_it...
Kevlin Henney has recently suggested we should stop calling it technical debt, and start calling it technical neglect instead.
Managed debt is a useful thing, if you ignore debt, you end up out of business.
Edit: Ok reading comments jogged my memory. It was something to do with the 'Sabre' system which was from the 1950's.
They run on a specialized version of Sabre that doesn't work the same as the other airlines on Sabre (AA, etc.) which is one reason you don't get SWA flights in Orbitz, etc.