If a city had a sign ordinance regulating taxes paid and permits bought and displayed for various types of signs, and if that sign fit into the regulated category, and if they did not have the proper tax/permit fee paid and properly displayed permit sticker ... it "could" be an "illegal" sign.
I know a person who had garage sale signs too close to the road in Pasadena, without a sign permit, and they were "illegal signs" even though there was in fact a Garage sale there so they were "valid" signs.
However, concerning 30.06, that has nothing to do with whether it is either "enforceable", or if it contains the proper language and correct citations/language (not 29ee like OLD signs) or even if it has been spell-checked.
So, I suppose in some particular situation an "illegal" "valid" sign (in violation of a city sign ordinance, not having a city permit fee paid etc.) might even be "enforceable" IF it was 30.06 "compliant"
(That said, expect to see me use the incorrect terms a lot,[because I have] but you know what I mean
)solne01 ... welcome to the forum, don't let our craziness scare you off. Personally, I'd have to see the sign to decide for myself whether it was compliant or not.... granted, a judge's opinion may differ from mine, but I'd have to see the sign to try and see if I would be breaking a law (which I wouldn't do knowingly) or not.