Good idea! I added a note about this in Notify bpo users once the migration is done · Issue #12 · psf/gh-migration · GitHub
People tend to report, comment, and follow issue when they are relevant to them. As time passes, they either find other workarounds and solutions, lose interest, forgot the context of the issue, or get busy with other things and this might lead them to ignore those issues (or in some cases actively unsubscribe, especially if they are a source of noise in their inbox). This doesn’t necessarily make the issue invalid: it might still affect people, even though they are not subscribed to the issue.
On the other hand, recently @guido shared an interesting article about noisy monitors (by Sam Schillace) that argues that the noise caused by those old issues might end up being more detrimental than the issues themselves.
Some time after the migration we can certainly review issues with no subscribers and decide whether we want to close them or not. This will also become easier if we set up an action to mark inactive issues as stale (even though, IMHO, the final judgment should be given by a human and they shouldn’t be closed automatically).