Notifications are being sent out by Google through Google Search Console with “mobile-first indexing issues detected” alerts. These emails indicates the issues Google has when it comes to moving the site over to mobile-first indexing. It also says “Google expects to apply mobile-first indexing to all websites in the next six to twelve months.”
Mobile-first indexing was first introduced in November of 2016. Half of all sites in Google’s search results were from mobile-first indexing by December 2018. Basically, all mobile-first indexing means is that Google will crawl your site from the perspective of a mobile-browser. It will use that mobile version for both indexing and ranking.
Notices are being sent out saying, “Google expects to apply mobile-first indexing to all websites in the next six to twelve months.” Here is a screen shot of this “mobile-first indexing issues detected” email from @KyleW_Sutton.
We know that Google wants to notify sites that aren’t mobile-first yet as early as possible, and give them specific advice on what those sites have to do in order to become mobile-first indexing ready.