As I understand it, Texas has had this law for some time, but there was an oversight in some legislative session the failed to specify a penalty. Consequently many PDs ceased to stop people for it. Here's the
link
FWIW, I was stopped stopped twice within a month or so around 3 yrs ago after Flower Mound got their automatic scanner, and then again in Farmers Branch. In both cases I told the officer that I had the plate under my seat and said I had no intention to mount it because it made my Z32 look like crap. I also told him if he wanted to write me up then I would sit there and make sure that he wrote up every other car that passed w/o one which I counted was about 1 in 6 at the time. Both muttered something about being able to arrest me if they wanted to; but then let me go.
I expect the new law will cause some to start aggressively enforcing again, esp. since they seem to love their new automatic scanners. I pretty much know exactly where I'll be stopped again; and not sure how I'll handle this time. At least I'm driving a different car now.