Google’s John Mueller said in a webmaster hangout on YouTube that the content of your web pages will not be indexed if it is only available in the desktop version of your web pages and not in the mobile version.
“[The mobile content on your website] doesn’t have to be exactly the same [as the desktop pages] but with mobile first indexing we will only index the mobile version of the page.
If there’s content on the desktop version that is important for you and it’s not available at all on the mobile version then we wouldn’t index that with mobile-first indexing. […]
Sometimes people write it shorter for mobile because it’s easier to read, sometimes they have smaller images or a different layout. That’s perfectly fine.”
You can watch the video here: