Actually, they don't HAVE to send you anything before they sue you.
It won't look so good at the trial, if you can prove that you requested it, and they didn't comply, but then, they can also provide the proof at the trial.
Here is a link to the Statute of Limitations listed by State.
Are you saying that without a doubt, you have NO IDEA what this debt could be for, and that you absolutely do not owe it, or are you just playing the "gee, I dunno" game to see if you can get away with it?
Take a look at these documents, if you actually receive them, perhaps they might refresh your memory of things.
Worst case here, is they do sue you, you both go to court, and the one with the best evidence, and the most proof will usually prevail.
Statute of limitations on debt collection