what do you do with code that clearly exists for a reason but nobody documented why?

working through an older service and there's a chunk of logic that's clearly handling some edge case that probably burned someone pretty badly at some point. no comment, the git history got murky during a migration a while back, the person who wrote it left.

curious what people actually do in practice. leave it alone with a 'don't touch, reason unknown' comment? dig through old tickets and logs to try to reconstruct it? slowly remove it behind a flag? something else entirely?