We actually published a research paper recently on COBOL communities, but I still don't have a great answer to your question.

Much of the information is locked away in developers' heads and it was difficult for us to find many resources on getting started with COBOL. In fact, most of the COBOL code on GitHub is homework assignments but almost no real-world programs that we could analyze.

Our paper which contributes a defects benchmark: https://austinhenley.com/pubs/Lee2022ICSME_COBOL.pdf