It looks like Google has killed off the Sitelinks searchbox that launched in 2014 and has been shown for many sites in the mobile and desktop search results. The sitelinks search box showed a search box in the search results snippet for some sites that lets you search within the site, like a site command but in the Google search results.
RankRanger's data shows the box has been completely removed from all of the Google results. I tested it by searching for terms like YouTube, Wikipedia, Pinterest and others, where you'd typically find the box but it is not showing up for me either.
Breaking: Google has removed the Search Box SERP feature from within the organic results globally - https://t.co/zdqvU8b7ue #SEO pic.twitter.com/ydrcovoKyM
— Rank Ranger (@RankRanger) September 27, 2017
The developer documents still say the feature exists, which makes me wonder if this is a bug. Maybe Google pulled the feature or maybe it is a bug, I emailed Google for clarity.
Here is what it looked like:
Here is what I get now:
Forum discussion at Twitter.
Update: Yup, as expected, this is a bug Google is working on fixing.