Not to derail, but this reminds me of a cave in Southern Arizona. Graffiti is banned and it’s heavily enforced, but there is ‘historic’ graffiti from the 1920-40s that is preserved. I always found this case very interesting.
https://www.nps.gov/subjects/nationalregister/how-to-list-a-...
Such graffiti is only an artifact when it's purpose, history and other aspects are lost to time due to lack of alternative documentation.
In effectively all cases these days there is simply no additional value obtained from graffiti like this, unlike that of street artists doing murals or painting trains.