There are two parts to the IT: the airline backend and the distribution backend. For the airline backends, Sabre used to build custom single-tenant ones per airline, on mainframes. The SWA one was very old, hence their inability to charge for bags. The Sabre distribution system is common of course, again on mainframes

Amadeus are eating them up, because their airline backend system is a shared multi-tenant setup, built on commodity hardware. Their distribution system used to be mainframes, but they managed to migrate away in the 2010s.

Sabre is still alive, but only in North America, and Amadeus is slowly chipping away (WN, AC..)