May not be illegal but as soon as a regular cop sees extra numbers by your plate you will be pulled over and it won’t be for praise on your ingenuity.
That goes for all the other things like diffraction gratings over the plate. Why give them a reason to pull you over? They can find something else to cite you on if they look hard enough.
About 4 years ago, daughter in another state sent me a plate rim that lit up purple. Had it maybe 2 years, when exiting the 101 freeway I was pulled over by Highway Patrol.
I REally didn’t know what for - couldn’t imagine.
Was told only 2 acceptable colors permitted, and 1 was NOT purple.
Had to say ‘bye’ to that fun license plate rim.
Some buggy laws pass that police are required to enforce.
And you’re right - they can always find something. We have a peace officer in family - right close by - who told me same many years ago!