John Mueller from Google recently confirmed testing of the mobile-first index in a hangout. The testing is being done in the live search results. The number or percentage of searchers being witness to these live test results was not explained, but it is not expected to be very huge. The Google aims to build new classifiers to debug and wants to ascertain how it impacts the current rankings and searchers.
As reiterated by John, the designing of internal classifiers is done to label the sites that have the corresponding desktop to mobile pages and which sites do not. This will help in knowing if any common issues are being noticed on the live web where they can make changes that need to be made by communicating with webmasters. Through direct communication, blog posts, Search console messages, etc., communication can be done with the webmaster. John didn’t clarify about the date of release but mentioned about the testing going on.