|An elven sage|
The problem with this, is that it assumes: 1) that the elves care, 2) that any elf even knew in the first place, 3) that the elves that knew and cared enough to notice are still alive, and 4) the living elves who know are traceable.
1. Caring. The elves generally don't really like humans, and view them with suspicion. The things the elves are likely to be interested in are questions like "are the humans planning any colonies in places we live, eg clearing forests for farmland?" not "where is the Invulnerable Coat of Arn these days?" or "What is the name of the guy that dug that megadungeon in the hills?". Only if these questions impinge on elven history are the answers likely to be recorded therein. Otherwise, it's all just "Some stuff the humans were doing but by the time anyone looked into it they had all died of old age. Typical."
2. Knowing. While there is a case that one may study those things that you are worried about, the standard game takes the view that what mostly happens is that the demi-humans keep away from the humans except for a handful of "exceptional" cases, where "exceptional" has strong undertones of "insane". This leads on to #3:
3. Living. Demi-humans who mix with humans in the "standard setting" are almost exclusively adventurers. And what do adventurers generally do better than anyone else? That's right - they die at low level trying to jemmy open a tomb somewhere and are never heard of again. So the elves, dwarves, gnomes etc. who are most likely to know stuff relating to human legends are the ones least likely to have taken advantage of those long lifespans.
4. Traceable. We have an elf who is interested in the subject at hand, has actual knowledge of it, and is still alive somewhere. Where? The answer depends in many things but unless you're running the City State of the Invincible Overlord it's unlikely to be "running the tobacconist shop at the end of the street". Even if you are running the CSIO, the chances are they're keeping their mouths shut about their background in order to not be constantly inundated by amateur archæologists who want some bit of pottery identified.
In the end, "ask an elf" is a viable solution but there's not really any good reason to think that it's an easy solution to questions of object identification and that's a good thing. Because it makes "ask an elf" into an adventure hook, not just a dull short-circuit of a mystery.