"What is your name?"
Translation:Vad heter du?
Thanks! I am not sure how far back "never" goes (I am almost 60) but I have swedish-english language books that are (maybe I should say were) pretty explicit on "ni" being used to address one with which one is not familiar. I take it that this is no longer the true. But if it was never the case then I think I have mistranslated an entry in SAOL under "du" which reads... "- ni med böjningsformer används också som singular i tilltal till mera obekant person."
Ni most certainly was a formal pronoun. The formal pronoun for the English "you." I'm an "elder" who was raised by Swedish grandparents in NYC, and was taught the "polite words" from a very early age. We would use "du" in the family to address elders and contemporaries, but "ni" to people new to us, strangers, and such anyone regarded as socially superior, such as the royal family, officials, and professionals. Perhaps social usage has changed, since a professional would also be addressed by their "title," such as "Herr Ingenjor" Johansson (with an umlaut over the o of course!).
This is off topic, but I have lived throughout the southern United States my whole life and the closest I have heard to that is "What are all y'all's names?", this being a contraction of: "What are all of your names?" Thank you for the clarification on the Swedish though, as I marked "Vad heter ni" and was confused when it was wrong!