At the moment, people are voting to close. But could we protect it instead (basically, a banner appears that says it's a historical question but no longer a good fit, and you can't answer or vote any more)? I think the question is still useful, but I agree that it shouldn't be left open.
What does it need protection from? Why would it need to be locked?
You might be operating under a misconception: closing a question is not an indication it's going to get deleted. That question isn't going anywhere. It's going to stay where it is. You will still be able to reference it. The only difference is that it will be closed, which we both seem to agree is appropriate.
You're right it's useful, and that's exactly why it's going to stay. Let me quote the guidance on the deletion feature us 10k+ users unlock:
When should I delete questions?
Closed questions that are of no lasting value whatsoever should be flagged and deleted.
Before voting to delete, please check that there are no good answers; if so, then the question should be flagged for moderator attention as a potential merge candidate. We don't like to lose great answers!
Emphasis not added; that's bolded in the guidance too. Would you say the question is of no lasting value whatsoever? I wouldn't either.
What about automatic deletions?
It's safe. Questions only get cleared away if they've been closed for two weeks, and have no answers, and the question is scored 0 or lower. This question doesn't meet those criteria. There's other circumstances, like if a question or answer accrues 6 spam flags, but that's not going to happen either.
Consider that the third question ever asked on this site was closed last year and is still around, and Stack Overflow has a question closed in 2008 which is still around.
Don't fret about closures. This doesn't and didn't need locking.
In fact, by locking this question we agree is useful, it can no longer be updated as a reward, we can no longer leave comments to request improvement, and we can no longer edit the answers to improve them, such as by keeping them current or updating broken links. Compared to closure, we might agree that locking this is worse!
Can we remove the historical lock on this thing and just let it be closed, since there was nothing wrong with it being closed to begin with?
(Unless, of course, the lock is completely appropriate for another reason, in which case let's leave it as is. But I'm pretty sure this is fine just leaving closed.)