I've heard a lot of them in songs like Three Ravens or My Son John but it isn't something you hear any more. What was the purpose and why did it die out?
Folk Songs of the Catskills by Cazden, Haufrecht & Studer touches on non-lexical vocables. It's hard to pinpoint origins exactly because it arose through folk tradition, but the derry down was a folk dance move dating back to at least sixteenth century England, but it's unknown whether the dance move was named for the song refrain, or the refrain was named for the song.
This source also suggests that the interjection of hey! may come from the Hay, another folk dance move.
The 1900 book Shakespeare in Music: A Collation of Chief Musical Allusions in the Plays of Shakespeare, with an Attempt at their Explanation and Derivation, Together with Much of the Original Music attributes "Hey down derry" to a Druidic origin, as well as discussing "Hey, trolly lolly".
I would argue with your assertion that you don't hear non-lexical vocables or "nonsense syllables" in music now. Even as I wrote this comment, I was listening to [Water Fountain by tUnE-yArDs] (https://www.youtube.com/watch?v=jbiFcPhccu8), the refrain of which goes
No water in the water fountain
No side on the sidewalk
If you say, "Oh my dear!
Whatchu doing now
Nothing much to do when you're going nowhere
Woo-ha!
Woo-ha!
Gotcha
We're gonna get the water from your house (your house)
and I don't think that song is an outlier. I'll keep my ears open, but I thinknon-lexical vocables are alive and well in popular music, they just might be "yeah, yeah, yeah"s, "ooooh baby baby"s and "nah nah nah nah nah nah"s.
Hi, this won't answer your question, but your post reminded me of this one, which you may find interesting
some of the nonsense syllables came from:
-bird sounds (turelurelu, saderaladon)
imitation of Latin chant/corruption of Latin chant (peri meri dictum)
corruption of texts in songs in other languages (for example, Byzantine chant)
lullabies (lulley, lullay, lulla, ninna, nonna)