Google's John Mueller announced at the Pubcon Pro Virtual 2020 conference that as of March 2021 - a firm date that will not be changed - Google will completely drop from its index all websites that are desktop-only. And m-dot sites - sites where mobile content exists on a subdomain - "may experience bugs".
In addition, Google will drop from its index any images or other website assets that exist only on the desktop version of a site.
Mueller said also that Google is now "almost completely" using the smartphone version of Googlebot for indexing.
And "...anything you want to have indexed...needs to be on the mobile site..."
If your website isn't mobile-friendly, fix that now!