I mean, they can be tagged walls, sure. I say this because these Soviet reliefs were often on walls or retaining walls. Here calling them walls is kind of like calling billboards walls ;) But I don't oppose it or anything, it's not wrong as such.

I would put that name into description. That's like calling a playground "Bērnu rotaļu laukums". But it doesn't really hurt anyone since no map knows what the hell `historic=relief`s are and won't show anything useful anyway.