Banking IT engineer here.
In our case, everything ‘core’: checking and savings accounts, loans and credits, credit and debit cards… anything requiring a sub-second response time while being bombarded with tens of thousands of transactions per second AND requiring strict ACID transactions end to end AND 24x7 availability with quick recovery in case of disaster.
Secondary stuff is being moved to other architectures. And new core stuff is being written in Java… and ran on the mainframe.
Not just games. Full operating systems from the 60s and 70s are being kept alive by hobbyists. Unfortunately there is no law or rule about proprietary/company specific software. In 50 years (or less) historians will know more about how the Romans did banking than how it was done in the early days of computing.