I see from my online research that ACP eventually became TPF (Transaction Processing Facility). (I have to wonder: Did someone think that was a more exciting name?) Parts of it are still in use, at least at United Airlines and Lufthansa, according to this undated press release (I warn you, it's full of the worst sort of IBMese, such as: "...its own continuous innovation capabilities could leverage common services, data, and functionality to create uniqueness and value" instead of saying, "Good grief, folks, it's 40 years old and time to upgrade!"). I even wrote about the transition of ACP to service-oriented architecture (SOA Migration: An Airline Keeping Its Feet on the Ground) largely because I liked how the project manager admitted ACP's legacy problems.
I don't know if anyone else is trying to collect the earliest examples of open source applications. I like to think so. But I would sure hate to think about this one's history fading out of memory.