In an announcement by Google, all new websites “previously unknown to Google Search” will be indexed using mobile-first indexing starting on July 1. Older webs sites that aren’t being indexed using mobile-first indexing won’t be switched yet to mobile-first indexing on July 1, but will be moved when those sites are ready to be moved to mobile-first indexing.
Google wrote “mobile-first indexing will be enabled by default for all new, previously unknown to Google Search, websites starting July 1, 2019. 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.”
Any sites that have moved over to mobile-first indexing will continue to be indexed using mobile-first indexing. Older sites that haven’t been migrated will continue to be indexed through desktop-first index, until those sites are ready.
“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,” the company said.
Google won’t be sending notifications to new websites since that will be the default state. “Since the default state for new websites will be mobile-first indexing, there’s no need to send a notification,” Google said.
So, what exactly is mobile-first indexing?
Basically, mobile-first indexing is how Google crawls and indexes the web. Rather than looking at the desktop version of the page, Google looks at the mobile version of the page. Another way to say it is, Google is is crawling and indexing a web page based on how it renders on a mobile phone versus a desktop. Over 50% of what Google indexes is indexed over mobile-first indexing.
You can also learn more from this Google help document and more articles on this site.