On Tuesday, May 28th, Google announced that new websites, “previously unknown to Google” will be indexed using mobile-first indexing by default starting July 1st. Google said in their statement, “It’s fantastic to see that new websites are now generally showing users—and search engines—the same content on both mobile and desktop devices.”
The change is set to occur to only brand-new websites Google isn’t aware of. Other websites that have not yet moved over to mobile-first indexing will continue to undergo indexing the traditional way: “desktop-first”. Google states, “For older websites, we’ll continue monitoring and evaluating pages for their readiness for mobile-first indexing, and will notify them through Search Console once they’re seen as being ready.”
Although Google will notify you when your site has moved to mobile-first indexing, they recommend checking your website by visiting the URL Inspection Tool in Google’s Search Console.
To learn more, see Google’s announcement here.