I was wondering the same thing, but Google street view (taken in 2014) doesn't show any evidence of such a big dip. How is that possible?? The only thing I can think of is that it's more of a "sag," where the middle has sunk 18" but the edges haven't moved much, but I'm not really satisfied with that theory.

https://www.google.com/maps/place/Millennium+Tower+San+Franc...

It's extremely unlikely that the building is sinking and the surrounding area is stable. More likely the building is pulling the entire area down, so it's probably hard to detect as a pedestrian.

A commenter upstream reports seeing the sidewalk being repaired multiple times throughout the last 10 years as well.

Wouldn't the width be a more appropriate measure than height?

Oh. I read it as the one corner was two inches higher. Vertical displacement / width. But that seems wrong now.