You unlock a character for spraying all tags across the 5 boroughs in the game, however this functionality is tied to achievements which to not exist on GOG.

This is just a PSA to everyone wondering. I've raised it with the devs on the Steam ver. forums.

Also, the functionality is simply not implemented on Switch. Will be addressed in the first patch, according to devs.

Ah. That would explain it. So all we need are achievements :D