I saw a whole corp POS platform a couple of decades ago that was hanging off a TFTP server on a machine that no one dared turn off in case the world ended. One day the DC UPS failed, it didn’t come back up and they had no retail operations for several hours while they sent a bunch of cash to a guy who had left to help them fix it.
There’s stuff like that everywhere lurking in the archaic.
I know of a modem in a DC which is used to talk to a branch office running AS400 hardware that is so old they have to buy spares off eBay.
My father worked as a banker for most of his life and when he was in his late twenties he got a position to oversee a smaller investment bank. This is sometime in the late 90s. When he started, he took a general look around, checked with everyone how things are going and happened to meet on of the few IT people working in the building. When the IT guy realized that he was speaking to a new person who might be able to change things around there, he was elated and told him that there was an issue the previous boss never took too urgently, even though it was quite critical. Apparently the servers that were running pretty much all of the transactions of that investment bank were located in the basement of that building and have literally never been migrated, upgraded or anything else. The servers that were left over from that time was literally one running machine and another machine that had died a few years prior that was now only used for spares in case anything on the singular still working machine broke. Since the hardware was so old, there apparently weren’t many replacement parts left and the ones that were left were incredibly expensive due to many bank depending on those specific servers.
Anyway, my father heard that story and immediately got the guy the funding he needed to migrate to a newer and better system. Sometimes I think about this kind of stuff, we think banks are really resilient (and they try to be), but I wouldn’t be surprised if setup like these still exist somewhere because people are too scared to touch them.