Hacker News Reader

> But AFAIK OSM doesn't have anything remotely like that level of sidewalk shape data.

Yes, for now only tiny part of sidewalks has area info. See https://wiki.openstreetmap.org/wiki/Key:area:highway

> I don't think outdoor building stairs are mapped.

It can be mapped, also not done in most of places

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.