Google announced this morning that starting on July 1, 2019 they will begin to index new web sites, not yet discovered by Google, using mobile-first indexing by default.
Old web sites not yet in mobile-first indexing still won’t be switched until they are ready but brand new sites, not yet indexed by Google, will come right out of the gate as using mobile-first indexing.
Google said that their “analysis has shown that new websites are generally ready for this method of crawling.” That being said, “mobile-first indexing will be enabled by default for all new, previously unknown to Google Search, websites starting July 1, 2019,” Google added. If you launch a web site that doesn’t work on mobile – then you will be out of luck and have issues.
Google won’t send notifications to these new sites that they are being indexed using mobile-first indexing because that will be the default behavior. So don’t expect to be notified when you launch a new site after July 1st that your site is using mobile-first indexing.
With older sites that have not yet moved, which is less than 50% of the sites Google said some time ago, those will continue to be using desktop-first indexing until they are ready. Google wrote “For older websites, we’ll continue monitoring and evaluating pages for their readiness for mobile first indexing.” And when those sites are moved over, Google said they “will notify them through Search Console once they’re seen as being ready.” Just note, that last half will be hard for Google to move.
Google is highly recommending that new sites go with a responsive design. Google wrote “While we continue to support responsive web design, dynamic serving, and separate mobile URLs for mobile websites, we recommend responsive web design for new websites. Because of issues and confusion we’ve seen from separate mobile URLs over the years, both from search engines and users, we recommend using a single URL for both desktop and mobile websites.”
Again, mobile-first indexing is not a ranking factor – it is just how Google indexes. Being mobile-friendly is a ranking factor and those are two different things.