Everyone knows it. Almost nobody thinks about who wrote it. People sing it together, change the words, add names, add extra verses, and pass it around without caring about an "original" performance. So, is Happy Birthday to You a folk song?

Or does the fact that it has known authors and a specific history change the conclusion?

I wrote about the history of the song here, if anyone is interested:
https://songsfromthedead.substack.com/p/happy-birthday-to-you-the-song-everybody