I was thinking of going through Meta Stack Overflow finding questions about the “too localised” closed reason (Why was my question closed?, etc.) and flagging them for closure as Off Topic > can no longer be reproduced. And then it occurred to me that this might not be a productive use of anyone’s time. Should I? Would this be a useful thing to do?
It wouldn't be useful at all, I'm afraid.
In general, trying to keep up with making sure everything here on MSO is still 100% relevant is just a losing battle. I wouldn't worry about it. There may be some benefit in closing unanswered questions about features that no longer exist, but even then, it's mostly just busywork.
If you come across a question that needs resolution and you feel like acting on it then, that's fine. But otherwise I'm sure there are far better ways you could spend your time such as answering some current support-type questions or participating in the ongoing feature/policy discussions.