TIL a useless structure preserved as part of a building or environment, becoming a piece of art itself, is called a Thomasson.

I capitalised it in the title because of its etymology, and for emphasis. I think I would use lower case in regular usage.

More examples:

https://www.messynessychic.com/2017/01/18/the-inexplicably-fascinating-secret-world-of-thomassons/

https://kyotojournal.org/kyoto-notebook/kyoto-tomason-the-hunt-for-hidden-hyperart/

https://www.amusingplanet.com/2017/04/thomasson-architectural-relics-that.html

https://commons.wikimedia.org/wiki/Category:Hyperart_Thomasson