Hi Bonnie!
My understanding is that the rule regulates the sign size and color, to keep things uniform and prevent the signage from being gaudy or tacky. It does not affect the yard signs, but does address the directionals we put up at the corners to alert buyers to homes that are for sale and to open houses. The specifics are probably on the Baltimore County web page, in the zoning section (link below).

I"ve never heard of such a thing unless this is something new. It is important to put one's name on the sign.
Perhaps it is a particular real estate company. Let me know if you find out one way or another. I sold in Balto. Co. for a few yrs. and this never existed.