It is in the word endings of "Novi Eboraci".
I think the nominative case of New York is "Novum Eboracum", so this is one of the other cases (possibly ablative or locative, but I'm rather rusty, so don't quote me on this).
It's the locative case ending (-ae for 1st decl fem, -i for 1st decl masc) which was used all the time in Old Latin to mean in/at but later came to be used mainly just for cities, with the word 'in' + the ablative used elsewhere.
Anybody tries "Quid est in Novo Eboraco?" (ablative of place) Does it work with Duo? Would it be acceptable?
According to the Tips and Notes, Novum Eboracum is the nominative (it's neuter) and Novi Eboraci is the locative case. See https://www.duolingo.com/skill/la/places/tips-and-notes
I see, but I just wonder about another way to express "location" using "ablative of place" which is actually more common than the use of "locative case". But here the ablative case hasn't formally introduced yet. But you have seen them with 1st declension word already, like "in America" or "in Italia". But I think "ablative of place" should be accepted anyway, because it also the very correct way to express it.