Google announced in a blog post yesterday that from 1st July, indexing for all new websites “previously unknown to Google Search” will be through the mobile-first index.
Google said they were taking this step as their analysis showed that new websites were generally ready for mobile-first indexing.
“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!”
There’s no change for older sites. Those already moved across to mobile-first indexing will continue as now.
Older sites not yet migrated will be moved over when ready. Google will continue to monitor and evaluate them for their readiness for mobile-first indexing and will send a notification via Search Console once they’re deemed ready and moved across.
Notifications won’t be sent for new sites as the default state for these will be mobile-first indexing.
You’ll be able to check for mobile-first indexing using the URL Inspection Tool, which will show you how your site was last crawled and indexed.
Google started rolling out mobile first indexing in March 2018 and now uses it for over 50% of search results globally, reflecting the continuing growth in mobile traffic and the move away from desktop.
Google says it continues to support responsive web design, dynamic serving, and separate mobile URLs for mobile websites, but recommends responsive web design for new websites and a single URL for both desktop and mobile.
You can see Google’s extensive guidance for getting websites ready for mobile here.