Comment by skissane

Almost certainly not. There are heaps of books in libraries which nobody has scanned yet, including many on COBOL. No LLM has read those.

Whether that makes a difference depends. If you are dealing with mainstream modern COBOL (like recent versions of MicroFocus or IBM Enterprise), it probably won't. If you are dealing with some obscure legacy COBOL dialect, odds are high there is some very helpful printed book which nobody has scanned.