I learned this week that Google will stop indexing desktop sites in March 2021

This means your revenue from Google searches will drop due to any content that you have on your desktop site which is missing from your mobile site. The solution is to make sure that all content which helps your SEO is also available on your mobile website.

You might think that you’ve only got one website and it is just mobile responsive. Because Google indexes only what the mobile browser shows, this means you still need to ensure the elements that are set to show on mobile meet your SEO needs.

Picture size matters. If you’ve reduced your mobile images for faster load times, these low-res images will rate lower, and will hurt you in the image search results.

If you’re concerned about the extra content and larger images hurting the responsiveness of your mobile website, it’s time you committed to a PWA solution. Progressive Web Apps preload pages that you might go to next. PWAs also retain visited content to provide a snappy user experience.

For more info on mobile-only indexing, see the Google Developers Article: Prepare for mobile-first indexing (with a little extra time)

Note that they keep using the phrase “mobile-first indexing”, but go on to say things like “We will only get the information of your site from the mobile version.” So when you read “mobile-first” think “mobile-only”.