I did some googling about this. As best I can determine, in CA altering the front plate has been legal since about 2014, subject to some restrictions on legibility. Altering the back plate in any way is flat illegal. As is covering it or obscuring it in any way.
Of course we all know enforcement of this stuff is negligible in every state. Hence my other thread about custom-made or altered official plates becoming commonplace in FL and asking about it elsewhere.
This may very quickly get like the crazy levels of speeding and aggressive driving on freeways where so many people are doing it that it becomes practically unenforceable.