National Manual of Uniform Traffic Control Devices of which each state pretty much adopts with there own title, is often not utilized very well by the more local entities. It should be the traffic engineers bible and it is most places.
It is not good practice to install street placards on a regulatory sign like a stop sign. Only additional placards allowed on such signage would be like a "4-way" or "cross traffic does not stop" placard. Also there are sign stardards to size too! And it’s height and distance from stop bar ( for a stop sign).
But,
You see all manner of such combinations in the county or small city roadways. Some places the sign fellas have pretty vivid imaginations. And have never heard of the NMUTCD. Or even their states version.
Edit,
I will add this caveat that a lot has changed since I retired. So I’m going on old accepted practices.
Keep preaching. A truck driver buddy of mine got his tickets dismissed because the "no truck" sign was 200 yards past the intersection, and he was pulling a set of doubles. Like he was going to back that up 200 yards and then around the corner.... Took him about a year, but the judge accepted the "non uniform sign" argument.
