The travel industry (esp the airlines) are moving to puzzle piece style integrations -- I know that Hilton Uses Sabre for incoming GDS reservations, but, uses salesforce internally for managing a lot of the guest interactions (including bookings and customer support): AA (as mentioned previously in this thread) uses multiple commercial systems, and Marriott uses a mixture of FOSSE, MARSA (there might be an H in there, but, it's been a while since I've been at MI) that talk to their backend microservices for their .com system.

MI picked up a LOT of technical debt and a LOT of security bugs when transitioning SPG programs and properties into MI's portfolio (thankfully, I was off of that project at that point in time).

I don't think this is the case where the FBI or other conglomerates have direct SQL-style access into their systems, but, more-so where FBI has retired or plans internally to pull data from systems when requested: When it's hard as hell for employees with the proper need-to-know for their application to pull up data in a meaningful fashion, you know that it's next to impossible for Law Enforcement to have a nice little dashboard where they can just type "Ian Wilson" and get a list of every place I've ever stayed ever (unless they're working with VISA: that's something that I kinda expect, tho).