So I've just spent some time looking into this - there are Cobol compilers that target JVM and .NET. GnuCOBOL compiles to C too.

Apparently 80% of all bank transactions and 95% of all ATM transactions in America are still running COBOL, as is the Social Security Administration and DoD's finance systems (https://www.youtube.com/watch?v=TBs7HXI76yU)

I read up on that a year or two ago. Then it was mainly IBM mainframes running z/os.

I work at a place where they use this for running legacy, but quite profitable, trading and payment systems. These were ported from both AS400 and mainframe alike and currently run on Microfocus on regular windows VMs on our internal cloud platform.

x86 / VSphere ESXi / Windows 2016-19

Would you happen to know why your company was able to ditch mainframe iron? (Was it because you didn't need the uptime, for example?)

It was an initiative from our group CIO to reduce the number if different architectures.

Uptime requirements did not change. There was just a very strong desire to ditch the mainframes ... and a long and painful way there.

I've always had the impression that they run on huge room sized mainframes in the bowels of banks and airline infrastructure but I have no idea how accurate that is today.

well they still run on the mainframe, many of which can sit under your desk nowadays like a standard tower. but there is a whole industry about replatforming and refactoring these apps to modern infrastructure and software stacks.