> Graffiti, fundamentally, is a defacement of public property as a form of artistic self expression.

The "Musée de la mémoire des murs" (museum of walls' memories), previously named "Musée des Grafittis historiques" sees grafittis much beyond your strict definition. https://www.verneuil-en-halatte.fr/musee-de-la-memoire-des-m...

I visited it long ago, but I remember drawings by prisoners on cells walls. Definitely not public property (or not visible by a wide public from public places).