Are frames prohibited?
Good question.
Here in Texas, I’ve read of cops giving out tickets because the frame was too big and it blocked a portion of the numbers.
No, but they have strict rules about them. They can’t obscure the state name even slightly, which is typically between the top mounting holes, so the frame end up having to be pretty minimal. Stupid law if you ask me, because half the states write the state name in some God-awful trendy font that you can’t read anyway. But we dare not obscure it, even partly, because the cops want to be unable to read it because of the font, not because of the frame.