No reg against it. They can be removed and holes filled with something solid if you like. Not all day cabs are built with back windows anyways. They are usually an option in class 8. Class 7 and lower medium duties are more common to have them standard due to all the different vocation options they are used for. I had a 24’ box behind the cab on my straight truck so the window was useless. All it did was let the sun reflect in and burn me and everything in the cab and make more heat for the A/C to fight to stay cool. I covered it with 4 layers of Limo tent and a beach towel that was the same color as the headliner.
As far as I know, as long as you have a mirror on bith sides of your vehicle you are not required to even have a rear window. The back windows in my Carry All have been covered by a curtain since about 1974 or so.
I don't see why not. However, it's gonna get annoying real fast though when you have to "cheat" during a blindsided alleydock or double check your lines before disconnecting your traile and now have to fumble with a flag or shade.
Thanks to all of you. That's exactly what I thought too. My coworker was trying to tell me the other day that a driver he knows was hassled about it when going through the scales and it didn't sound right to me.