Everyone (including me) loves to give IBM shit, but they do actually know what they're doing when it comes to this stuff. There are many lessons and examples from mid-range and mainframe servers from IBM that are either ignored or re-invented in modern tech.
The codepage was very specific, it was not part of the standard Java JDK but it was included in the IBM Java JDK.
Small nitpick: they have been maintained since 50 years ago. Very few programs written 50 years ago have spent the last 10 years without some sort of patch.
My grandfather made good money in the late 1990s patching up ancient systems for the y2k bug, before his retirement.
Maybe it's time to force the banks to replace code they cannot maintain.
There's no way to be fully confident that what appears to be a simple update might not break the whole system. And I'd imagine this is likely what the bank is wanting to avoid rather than having to pay programmers to implement the changes.
You only have to look as far as TSB for a cautionary tale.
Except it's not running perfectly fine. It's butchering customers' actual real names. That may have been acceptable in the 80s, but it isn't anymore.
There's also the case to be made that if the sources are lost, you need to rewrite from scratch, as you never know when you need to implement new functionality that business deems critical. If you only start the new implementation after business is breathing down your neck, you are in a world of hurt.