That was my first thought when I just saw "sidewalks" & "crosswalks" too. But AFAIK OSM doesn't have anything remotely like that level of sidewalk shape data. And while stairs on paths are mapped, I don't think outdoor building stairs are mapped.
Here, in OSM, I can see stairs shape, type of pavement, stairs direction, and amount of stairs. Not sure about sidewalks, though. OSM is pretty detailed altogether. It depends on user input, but the tools are there.
Stairs on Google are often the result of leveraging Captcha questions, i.e. Google made the public do the work but then locked up the data under a non-public license.
