I believe the majority of comments we receive on closed GitHub issues belong elsewhere, either as new forum topics, or new GitHub issues.
Call it a character flaw, but I find the behavior annoying, and tend to ignore comments on closed issues. I suspect others who monitor the Hugo repositories have similar feelings.
Instead of ignoring these comments, and the people who took the time to write them, perhaps we can automatically lock issues N days after they are closed. I’m not sure what N is, but I’d be tempted to start with a low number (7) and increase it as we gain experience.