This was sudden.
Personally, I knew nothing of this until I saw them painting the street on Monday.
Due to the success in New York of creating public plazas, San Francisco decided to try to copy the idea.
The new plaza extends down 17th Street from Castro to Hartford. The F-Market stop still stands right in the middle of it, so public transportation will parade through the middle of this plaza every 6 to 10 minutes.
According to the Chronicle article about the project, the "benches" in the picture are "excess pieces of granite used by the city for curbs." The "planters" look like steel drums that have been painted tan. In other words, it looks like they spent as little as possible on this upgrade, which I guess makes since if it's only a temporary test -- much less money to lose if it doesn't work and they have to take it down.
When I took these pictures this morning, they were setting chairs and tables and a sound system for the dedication ceremony that was to be held at 10:00 this morning. And, apparently, there is to be what they are calling a "Community Celebration" this Saturday, May 16, starting at 12:30 p.m.
I highly recommend that you check out this article at SF.Streetsblog, which includes a nice, detailed drawing of the plaza plans.