Is there any point to upvoting a feature request that has been given status-declined in the past?
Is there any chance that with enough upvotes it will actually be reconsidered?
Yes, status declined has been reversed on several occasions. As noted in my answer to this question about supported old requests, there are incidents that even long old declined requests ended up implemented.
Vote it to show support, also make sure that if you can provide ample evidence that the outlook should be reconsidered, that you do so.
It's always worth upvoting.
If there hasn't been much activity in the past, it might also be worth leaving a comment, or even an answer outlining why the request is still relevant, and should be implemented.
Declined feature requests have been revived and implemented in the past - not often, but it has happened.
If the request is something that is contrary to the "spirit" of what we're trying to do, it's probably declined forever.
In other words:
Everything else is debatable, but it's up to you to decide which requests are fundamentally at odds with our philosophy of Q&A.