My argument for Yonkers being upstate is that at one point Uber was not allowed to go there and it had a casino, whereas the five boroughs didn’t. I still use that as my line. If I’m going to be less pedantic about it, I think Poughkeepsie might be the line simply because that’s how far the reach of the MTA extends.

My definition of Upstate has always been if you can't get there via Metro-North, it's Upstate. (Though as an UWS resident I'll humorously tell my friends who live in Harlem and Brooklyn that anything north of 110th Street is Upstate and anything east of 1st Ave is Long Island)

As someone who lives in Manhattan, grew up in Rockland, and has spent a lot of time in Westchester…. Upstate is anything north of Poughkeepsie.