Before Google made a massive index change in 2009 or so named Caffeine, Google released a public sandbox for you to preview the changes in the search results based on this index change. In short, it gave people like you and I a way to see how the new index will impact the rankings in the Google results before it went live. It also gave Google a way for them to get feedback from us on the new index before the real Googler users saw it live.
John Mueller of Google was asked if we would see the same thing with this new massive index change, the mobile-first index. John Mueller said no, "I don't see that happening," he said. He said he would bring the idea to the team though.
What was interesting is that when Google did announce the mobile-first index, they said they launched it on a very small level to a small percentage of searchers. That is how Google typically tests these roll outs now. But maybe it makes sense to do a public preview so us webmasters and SEOs can give Google some straight feedback?
I don't see that happening, but it's an interesting idea worth discussing with the team.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) June 2, 2017
Forum discussion at Twitter.