This question was downvoted and closed, but I would like to know: How is it significant?
It has a historical lock because a former moderator placed the lock there. I don't know why he did it. I can't even begin to speculate. But removing the lock would change little: The question is terrible and the user isn't going to fix it.
There's probably still room for a good question along these lines, but this wasn't it. No longer seen in this post are several deleted comments where the poster explicitly stated it was a "test" question and people reacted quite negatively to that. Then he claimed it was based on a real situation, but refused to clarify it so that he could get a proper answer.
I'm pretty sure we'd lose just about nothing if the question were deleted. If anyone else has seen it since the week it was posted, I'd be surprised.
A few things stand out. First, the ex-mod that locked it, and the ex-mod that answered it. Part of the reason that the ex-mod who locked it is an ex-mod is due to actions like this. If you look hard enough, you'll find hundreds of similar historical-locks that don't quite make sense.
As suggested in comments, it was locked to prevent it from being bumped to the front page ("active" questions view) when I retagged it. At least as of when I last had diamond privs, unlocking a question was one of the actions that bumped a question to the front page, so it stayed locked.
No sense in stirring that turd back to the top that I could see, but perhaps the current mods will feel differently.
The question is not very valuable in its current state. There is however one reason I won't vote to delete it. Deleting the question would also delete the answer.
The answer which was given to that question is a good answer. And that answer would also be applicable to more realistic questions that might come up. So there is a possibility that somebody may search for an answer and find that answer useful.
But it is not the question and answer I would consider to deserve the historical question notice. The scenarios where that notice would be most applicable is good questions with good answers which just happens to be off-topic according to our current standards.
The notice has since then been removed, and I think that is all that needed to be done about that question.