I come from a different tradition where the no known author definition would be useless. I don't know why the difference, but the authors of our even more than 500 years old folksongs are often known. Noone over here would for instance not call Die Lorelei a folksong just because the author is known.

It is a folksong because I knew it and could sing it before I even cared or knew whether the song had a known author or not. I learned it orally without any recollection when and from whom. If that is true for a song for a significant percentage of the population it is a folksong in my mind.

The no known author definition has one more disadvantage in my mind: If a researcher would find in an obscure library an old document giving the name of the author to a song that previously had no known author would that mean that that song has to be removed from the folk song corpus henceforth?

Wolfgang