You can, actually:

https://news.ycombinator.com/front?day=2007-02-20

You can. If you hover over you can see the ID # of the thread - copy that ID # into the url of this thread and it'll direct you right there. Case in point: https://news.ycombinator.com/item?id=189

They're still available, just note the id. For instance the top post on that day: https://news.ycombinator.com/item?id=227