I finally made it up to the other end of the mysterious block of Oregon that’s blocked by a historic retaining wall.
There’s a beautiful church up there, but no real strong evidence that would have necessitated a blocked street.
I’m leaning towards the decision merely being the choice of the developer, who filled this block in late in the development of the area.
Update: The fence has been opened, as of 2018.
Sadly, the crime that caused the staircase to be blocked off in the last couple of decades has started to cause abandonment.