I'm personally fine with it and would moderately encourage commenting on old posts. It actually serves a few useful purposes:

1. If the question/comment is actually useful to other people, then it's useful in object level terms to other people on it's own terms.

2. I think asking questions is often helpful for clarifying your own thinking, so even if it's mostly rough notes to yourself it still seems cheap and pretty fine.

3. It's actually kinda nice to periodically give older posts a bit more time on the spotlight, just to expose them to newer users or remind people of them.

4. I think it's specifically useful for LW to move in a direction where content is maintained and improved over the longterm.