Google has been working on mobile-first web indexing for nearly half a decade. Although 70% of all websites surfaced within search results today are optimized for mobile devices, that lingering 30% means mobile users still land on unwieldy desktop-only interfaces far too often. To ensure the best browsing experience for everyone, Google is switching to mobile-first indexing for all websites starting in September 2020.
The announcement serves as a warning to any web developers who have yet to implement mobile compatibility on their sites. For many antiquated pages, this can only be achieved by redesigning the website using Responsive Web Design (the Google-recommended option), dynamic serving, or designating separate URLs. After one of these methods has been implemented, Google then needs to crawl the new site with a smartphone Googlebot and update its search index before it can finally offer mobile results to users.
From now until September, Google plans to continue to activate mobile-first indexing on websites as they become optimized. The search giant didn't outline what will happen to non-mobile websites after September 2020, but it's safe to assume that they'll get buried in the long lost abyss of forgotten results.