Tag to use for permanent political signs

I agree. The word “advertising” to me means that the exposure for commercial activity/gain, i.e. “commercial propaganda”. If it is political in nature (like pictures by @adreamy probably are), “political propaganda” would make sense to me. If it is a mural with Jesus and angels or text from gospels, it is “religious propaganda”.

Any of those may be just plain text, or it may be graffiti, or it may be mural or some other kind of artwork. If it is artwork, at least there is Key:artwork_type - OpenStreetMap Wiki

it would be nice if wiki-templates like those worked in this forum !