Same as @t1m1t, when I was looking to have my Model Y tinted (April 2021), Signature gave me a quote for the front and back windows, as well as the windshield and glass roof. In my case, they never said anything about not tinting the back windows or hatch.

I had my prior car tinted at Signature as well, and did not have any issues. However, this time around, I got my Y tinted at Rodz in Fountain Valley, as my sister had her Model 3 tinted there (on the recommendation of a few other Tesla owners), and also because they loaned me their vehicle for the day (since the job took all day, and I could not be without a car). Rodz was also slightly less expensive than Signature – about $150.

EDIT: While Signature was slightly more expensive than Rodz, Signature has a No Fault warranty, where it will replace your tint if your window gets broken. When I spoke to Signature in 2021, they confirmed the No Fault warranty only applied to the side windows, and presumably the rear hatch, i.e., it does not apply to the windshield or glass roof.