No one knows but it has been that way for both the 19" and 20" Model Y winter tire and wheel packages since this past summer.
Tire Rack should be able to get you a compatible winter tire and wheel package. You may have to get creative in choosing a slightly larger winter tire if the OE size is out of stock.
19" OE size 255/45R-19, 265/45R-19 should fit
20" OE size 255/40R-19, 255/45R-19 should fit.
You can compare the dimensions of different tire sizes here:
Tire Size Comparison
Speed rating is not critical for winter tires as you would be unlikely to drive at the Performance Model Y's top speed of 155 MPH.
Y - 186 MPH
W - 168 MPH
V - 149 MPH
H - 130 MPH
T - 118 MPH
Tires must have a load rating of Extra Load (XL) and the Load Index should match or exceed the OE tires:
19" Load Index: 104 minimum
20" Load Index: 101 minimum
21" Load Index (Rear Tires): 103 minimum
21" Load Index (Front Tires): 98 minimum
Tire Rack