“I wouldn’t be surprised if there is some mainframe code from the 1960s still running part of that beast.”
Consider this verification: much of United’s code exists unchanged from the 1960s. And Sabre — the biggest airline scheduling and booking company still in independent existence, which started life as American’s reservations system — still contains code from the initial system, written on and for IBM 7090 mainframes in 1958–1960. And its output is still more or less designed for a 2260/3270 in its native mode, and its text is still all-caps fixed-column-width… and is still available in EBCDIC.
This is not to mention OAG, about whom the less said the better.