When I see rust on structural steel, it’s usually in a confined space. We find rust because the steel jacks apart the surrounding masonry or concrete as it turns into rust.
When steel is exposed, it’s easier to find the rust and it’s easier to prevent it from getting worse by scraping and painting. Exposed exterior steel structure is rare in buildings, although relatively common in bridges.
What we do find on a lot of buildings is exposed non-structural steel and wrought iron:
That’s a vertical bar in a security grille on a building in SoHo. The bottom, just above the horizontal tie plate, is full size; the narrow part above was once that size until it rusted. Roughly three-quarters of the bar cross-section is gone. Given a few more years, the bar will separate into an iron stalagmite just above the horizontal and an iron stalactite above. Fortunately, the failure to come will not make the building less safe.