In the case that someone makes a certain claim before a court but requires a witness(es), however, the witnesses are deceased. Does that mean the person's claim has no bearing?
In Shulchan Aruch, Choshen Mishpat 140:4 we learn that there is no difference whether your witnesses are dead, non-existent or unreachable.
In all these cases they are treated as if they do not exist.
That fact that you once had witnesses seems to have no practical application in Halacha.
We actually have a fascinating case in Ketubos 23a (bottom of the Daf) where some people knew that witnesses existed, but they were overseas. Somebody took advantage of this fact and got a lenient ruling as if there were no witnesses.