Just recently a script that has been running flawlessly has begun to return a 410 response to the HTML get call to both the http://www.zillow.com/webservice/GetSearchResults.htm and the http://www.zillow.com/webservice/GetDeepSearchResults.htm
Pasting the url that the script is fetching into the browser gives the same 410 error, as though they have removed it entirely.
I see references to their new Bridge API, but no notifications that their old API was being discontinued that I can find. Any insight?
Zillow has shut down all of their data APIs as of the end of February. As was stated in another answer, it appears as though they shut it down as a result of abuse, but did so with no mention of it at all (as a matter of fact, the documentation and sign up remain active to this day, nearly 3 weeks after the API began throwing 410 response codes).
In 2016 Zillow purchased Bridge Interactive, which is a company that specializes in Broker and MLS back-office solutions and data management. see press release here
Bridge Interactive now has the Zestimates API (see documentation here) along with a new API that appears to offer API access to public data too (Public Data API docs)
I had applied for access to the Bridge API in February using their web form on the Bridge Interactive Website shortly after the demise of the Zillow APIs and after about 2 weeks and a bit of back-and forth via email I just got my invite to access the Bridge API.
It does appear to have the same restrictions as the Zillow API just on their new platform. Sign-up is a manual process so it takes some time but, knock on wood, I might be back up and running in a short period of time after integrating with the new API (which, of course, is completely different than the Zillow APIs).
Update 2021-03-28
It appears as though on or about March 26th, their API began to respond again -- I have tested the getSearchResults and the Zestimate API and both are responding as they previously had.