From a quick search, there are now sites like [0] where you can seemingly buy road signs which adhere to MUTCD (I can only imagine it was much more difficult for Ankrom in 2001). From the customer reviews, it sounds like many are placed within personal property, such as indoor decorations or arrows directing delivery drivers.

However, I wonder how much trouble people get in practice for putting up signs on less-trafficked public roads (e.g., signs like "Slow, children at play" or pedestrian crossings). I'd like to think that a well-intentioned sign on, say, a quiet suburban street would be allowed to exist.

[0] https://www.roadtrafficsigns.com/