Unless something strange happened across all three reports, your credit reports should show all opened or closed credit cards from the past 7 to 10 years. Any closed cards older than that should have fallen off your reports by now.
The average American has two credit cards, total, on his or her credit report, so eleven from one bank is well above average by just about any standard.
As for Amex purging records, you've lost me there. Just how long ago did you buy this ticket? If United refunded to a closed card that was actually yours, Amex assuredly should have a record of that old, closed card. And if United somehow messed up and refunded the ticket to some other person's card, then there's obviously nothing for you to find in a credit report.