I asked my local Tesla dealer about this today. They admitted that they're not expecting to offer winter tires for the Model Y this year.
Their suggestion was that I buy winter tires on my own and have Tesla install them. Note that this would mean swapping out my 21" Überturbines for another set of 21" tires on top of the same wheels. And, yes, that means the different width in the back.
I was actually willing to get 19" wheels with 19" tires in a square configuration, but I'm not sure how practical that would be.
Interesting feedback you received. There's a part of me that tries to recognize that parts and service advisors have been put in a tough spot not having any answers or solutions from Hawthorne/Freemont to give to PUP owners in more than half of the country.
I ended picking up a set of barely used 19" Geminis and will just roll with the fresh A/S tires this winter (and get snows next year). May be a bit of a risk but it seems there are enough images and videos of folks who've gone this route and Tesla even testing with Gemini wheels and the PUP brakes. Receiving tomorrow and planning to put them on in about a week to confirm it'll be good to go.