I can't tell you how common it was or is, but I can tell you that I worked with at least two banks in Europe in the early 2000s that were rewriting core transac

I can't tell you how common it was or is, but I can tell you that I worked with at least two banks in Europe in the early 2000s that were rewriting core transaction processing and routing systems that were originally written in COBOL, and for which they had lost the source code. They patched the systems when necessary by patching the object code in assembler. The systems were 30 odd years old at the time. Nobody seemed to be able to explain how the source code was lost, other than that it had something to do with corporate mergers and acquisitions. The fact that they were spending millions to rewrite them was an indication of unsatisfactory they found the situation to be.