We first started working on mobile-first indexing a number of years ago. Since then, we've switched to indexing the majority of web pages with Googlebot-Smartphone. Our existing plan was to complete mobile-first indexing by March of this year.
In the meantime, after analyzing the sites that are not yet indexed mobile-first, we've determined that some of these sites are still not ready to be shifted over due to various, unexpected challenges that they're facing. Because of these difficulties, we've decided to leave the timeline open for the last steps of mobile-first indexing.
In Google Search Console, there are two ways to check for mobile-first indexing: the status is shown on the settings page and in the URL Inspection Tool. For websites that Google has already moved to mobile-first indexing, no changes are necessary. We'll continue to crawl and index your website with mobile Googlebot.
Google has announced that they will be gradually moving websites over to mobile-first indexing. This is a change in how Google crawls and indexes the content on a website, and it may result in changes for websites that have different mobile and desktop versions. Websites that have the same content on mobile and desktop are not likely to see any changes. Google has provided a guide for mobile-first indexing best practices, and they encourage website owners to resolve any issues their site may have before the switch is made. Over time, as more and more sites become ready for mobile-first indexing, Google will start shifting them over again.
Back in December 2020, Google announced that they would finish the move to mobile-first indexing by March 2021. However, they have since found that some sites are facing challenges that they weren't expecting. Because of this, they don't have a specific final date for the move to mobile-first indexing yet.
For questions about this process, you can post in the Search Central help community or contact Google through their online events.