Google is enabling mobile-first indexing (crawling sites with a mobile user-agent) by default for new domains as of July 1, 2019. These are sites Google defines as “previously unknown to Google Search.”
Mobile searches are the primary way that users engage with the search engine for the past few years, which caused Google to begin prodding website owners to make their content more mobile friendly. Google’s indexing and ranking systems originally used the desktop version of a page’s content, but this can cause problems when desktop and mobile sites contain different versions of content.
Mobile-indexing is not default for all websites yet, but Google started rolling it out more widely in 2018. Google evaluates older websites for readiness based on a variety of factors:
For existing websites we determine their readiness for mobile-first indexing based on parity of content (including text, images, videos, links), structured data, and other meta-data (for example, titles and descriptions, robots meta tags). We recommend double-checking these factors when a website is launched or significantly redesigned.
It’s important to note that there is no separate “mobile-first index.” Google still references a unified index for serving search results. The difference is in whether a page is crawled by a desktop or mobile user-agent.
Prior to responsive web design becoming the industry standard, having a separate mobile site was a common practice. Google is actively discouraging this approach now (although still supporting it), as it often causes confusion for users and search engines alike.
WordPress sites using a responsive theme shouldn’t have any issues with mobile indexing as long as they are not farming out their mobile sites to a separate domain. A responsive theme is usually enough to make a site compatible with this method of crawling.
Website owners can check to see how their sites are being crawled by clicking on the “URL Inspection” tab inside the Google Search Console.
Webmasters of older sites will get a notification from the Search Console when their sites are moved over to mobile-first indexing.
After successfully ramping up mobile-first indexing from a small-scale experiment a few years ago, making it the default for new domains should be a good test for evaluating how aggressively it can be rolled out in the future.