To owners of all websites. Google has just released the following statement.
Background: Mobile-first indexing means that Googlebot will now use the mobile version of your site for indexing and ranking, to better help our (primarily mobile) users find what they’re looking for. Google’s crawling, indexing, and ranking systems have historically used the desktop version of your site’s content, which can cause issues for mobile searchers when the desktop version differs from the mobile version. Our analysis indicates that the mobile and desktop versions of your site are comparable.
This means that you may see more traffic in your logs from Googlebot Smartphone. You may also see that snippets in Google Search results are now generated from the mobile version of your content.