Google has started highlighting search areas in Pink color. Is this feature available in Google Maps API 3?

No. It's not available in the API.

(It may be available in the future. Features of Google Maps do migrate into the API, but Google don't make announcements in advance and no-one has a crystal ball.)

You would need to find the city boundaries and draw the line yourself. Boundary data is almost certainly public-domain for the US. Other jurisdictions may not be so forthcoming.

Although a very old post my answer might help somebody out there as am landing on this post every time I query for bounding box:

Yes we can get the bounding box coordinates that google is using to highlight a region using Googles geocoding service. check for the response of the API call in the below sample from Google.

https://developers.google.com/maps/documentation/javascript/examples/geocoding-simple?refresh=1