In an announcement made by Google, by September 2020, all sites will be crawled and indexed using mobile-first indexing.
Google said, “To simplify, we’ll be switching to mobile-first indexing for all websites starting September 2020.” A majority of sites have moved over already. It’s possible to check if your site has moved over by logging into Google Search Console. More on how to do that over here.
“From our analysis, most sites shown in search results are good to go for mobile-first indexing, and 70% of those shown in our search results have already shifted over,” Google said. There’s a good chance that your site has already moved over to mobile first indexing.
If your site was not moved over yet, you may have already received a notice from Google with more information. Here is a screen shot of a notice: